drm/radeon/radeon_atombios: Move 'radeon_add_atom_encoder()'s prototype to shared header

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

 drivers/gpu/drm/radeon/atombios_encoders.c:2721:1: warning: no previous prototype for ‘radeon_add_atom_encoder’ [-Wmissing-prototypes]
 2721 | radeon_add_atom_encoder(struct drm_device *dev,
 | ^~~~~~~~~~~~~~~~~~~~~~~

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_atombios.c

@@ -37,10 +37,6 @@
 #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);
-
 union atom_supported_devices {
 	struct _ATOM_SUPPORTED_DEVICES_INFO info;
 	struct _ATOM_SUPPORTED_DEVICES_INFO_2 info_2;

drivers/gpu/drm/radeon/radeon_atombios.h

@@ -28,10 +28,14 @@
 #ifndef __RADEON_ATOMBIOS_H__
 #define __RADEON_ATOMBIOS_H__
 
+struct drm_device;
 struct drm_display_mode;
 struct radeon_device;
 
 bool radeon_atom_get_tv_timings(struct radeon_device *rdev, int index,
 				struct drm_display_mode *mode);
+void radeon_add_atom_encoder(struct drm_device *dev, uint32_t encoder_enum,
+			     uint32_t supported_device, u16 caps);
+
 
 #endif                         /* __RADEON_ATOMBIOS_H__ */