drm/radeon/radeon_legacy_encoders: Move 'radeon_add_legacy_encoder's prototype to shared location

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

drivers/gpu/drm/radeon/radeon_legacy_encoders.c:1746:1: warning: no previous prototype for ‘radeon_add_legacy_encoder’ [-Wmissing-prototypes]
1746 | 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_atombios.c

···
 #include "atom-bits.h"
 #include "radeon_asic.h"
 #include "radeon_atombios.h"
+#include "radeon_legacy_encoders.h"
 
 extern void
 radeon_add_atom_encoder(struct drm_device *dev, uint32_t encoder_enum,
 			uint32_t supported_device, u16 caps);
-
-/* from radeon_legacy_encoder.c */
-extern void
-radeon_add_legacy_encoder(struct drm_device *dev, uint32_t encoder_enum,
-			  uint32_t supported_device);
 
 union atom_supported_devices {
 	struct _ATOM_SUPPORTED_DEVICES_INFO info;

drivers/gpu/drm/radeon/radeon_combios.c

···
 #include <drm/radeon_drm.h>
 
 #include "radeon.h"
+#include "radeon_legacy_encoders.h"
 #include "atom.h"
 
 #ifdef CONFIG_PPC_PMAC
···
 #include <asm/pmac_feature.h>
 #include <asm/prom.h>
 #endif /* CONFIG_PPC_PMAC */
-
-/* from radeon_legacy_encoder.c */
-extern void
-radeon_add_legacy_encoder(struct drm_device *dev, uint32_t encoder_enum,
-			  uint32_t supported_device);
 
 /* old legacy ATI BIOS routines */
 

drivers/gpu/drm/radeon/radeon_legacy_encoders.h

···
 
 void radeon_legacy_backlight_init(struct radeon_encoder *radeon_encoder,
 				  struct drm_connector *drm_connector);
+void radeon_add_legacy_encoder(struct drm_device *dev, uint32_t encoder_enum,
+			       uint32_t supported_device);
 
 #endif				/* __RADEON_LEGACY_ENCODERS_H__ */