drm/radeon/radeon_legacy_encoders: Move 'radeon_add_legacy_encoder' prototype to shared header

Fixes the following W=1 kernel build warning(s):

drivers/gpu/drm/radeon/radeon_legacy_encoders.c:1745:1: warning: no previous prototype for ‘radeon_add_legacy_encoder’ [-Wmissing-prototypes]
1745 | radeon_add_legacy_encoder(struct drm_device *dev, uint32_t encoder_enum, uint32_t supported_device)
| ^~~~~~~~~~~~~~~~~~~~~~~~~

Cc: Alex Deucher <[e-mail]>
Cc: "Christian König" <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Sumit Semwal <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Lee Jones <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/radeon/radeon_encoders.c

···
 #include <drm/radeon_drm.h>
 
 #include "radeon.h"
+#include "radeon_legacy_encoders.h"
 #include "atom.h"
 
-extern void
-radeon_legacy_backlight_init(struct radeon_encoder *radeon_encoder,
-			     struct drm_connector *drm_connector);
 extern void
 radeon_atom_backlight_init(struct radeon_encoder *radeon_encoder,
 			   struct drm_connector *drm_connector);

drivers/gpu/drm/radeon/radeon_legacy_encoders.c

···
 
 #include "radeon.h"
 #include "radeon_asic.h"
+#include "radeon_legacy_encoders.h"
 #include "atom.h"
 #ifdef CONFIG_PMAC_BACKLIGHT
 #include <asm/backlight.h>

drivers/gpu/drm/radeon/radeon_legacy_encoders.h

···
+/* radeon_legacy_encoders.h -- Private header for radeon driver -*- linux-c -*-
+ *
+ * Copyright 1999 Precision Insight, Inc., Cedar Park, Texas.
+ * Copyright 2000 VA Linux Systems, Inc., Fremont, California.
+ * All rights reserved.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * PRECISION INSIGHT AND/OR ITS SUPPLIERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
+ * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+ * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ *
+ */
+
+#ifndef __RADEON_LEGACY_ENCODERS_H__
+#define __RADEON_LEGACY_ENCODERS_H__
+
+void radeon_legacy_backlight_init(struct radeon_encoder *radeon_encoder,
+				  struct drm_connector *drm_connector);
+
+#endif				/* __RADEON_LEGACY_ENCODERS_H__ */