drm/radeon/radeon_pm: Move 'radeon_pm_acpi_event_handler' prototype into shared header

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

drivers/gpu/drm/radeon/radeon_pm.c:74:6: warning: no previous prototype for ‘radeon_pm_acpi_event_handler’ [-Wmissing-prototypes]
74 | void radeon_pm_acpi_event_handler(struct radeon_device *rdev)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

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_acpi.c

···
 #include "atom.h"
 #include "radeon.h"
 #include "radeon_acpi.h"
+#include "radeon_pm.h"
 
 #if defined(CONFIG_VGA_SWITCHEROO)
 bool radeon_atpx_dgpu_req_power_for_displays(void);
···
 #endif
 
 #define ACPI_AC_CLASS           "ac_adapter"
-
-extern void radeon_pm_acpi_event_handler(struct radeon_device *rdev);
 
 struct atif_verify_interface {
 	u16 size;		/* structure size in bytes (includes size field) */

drivers/gpu/drm/radeon/radeon_pm.c

···
 #include "avivod.h"
 #include "r600_dpm.h"
 #include "radeon.h"
+#include "radeon_pm.h"
 
 #define RADEON_IDLE_LOOP_MS 100
 #define RADEON_RECLOCK_DELAY_MS 200

drivers/gpu/drm/radeon/radeon_pm.h

···
+/* radeon_pm.h -- Private header for radeon driver -*- linux-c -*-
+ *
+ * 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_PM_H__
+#define __RADEON_PM_H__
+
+void radeon_pm_acpi_event_handler(struct radeon_device *rdev);
+
+#endif				/* __RADEON_PM_H__ */