drm/radeon/radeon_encoders: Move 'radeon_atom_backlight_init's prototype to shared header

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

 drivers/gpu/drm/radeon/atombios_encoders.c:186:6: warning: no previous prototype for ‘radeon_atom_backlight_init’ [-Wmissing-prototypes]
 186 | void radeon_atom_backlight_init(struct radeon_encoder *radeon_encoder,
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~

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.h

@@ -28,14 +28,18 @@
 #ifndef __RADEON_ATOMBIOS_H__
 #define __RADEON_ATOMBIOS_H__
 
+struct drm_connector;
 struct drm_device;
 struct drm_display_mode;
 struct radeon_device;
+struct radeon_encoder;
 
 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);
+void radeon_atom_backlight_init(struct radeon_encoder *radeon_encoder,
+				struct drm_connector *drm_connector);
 
 
 #endif                         /* __RADEON_ATOMBIOS_H__ */

drivers/gpu/drm/radeon/radeon_encoders.c

@@ -31,14 +31,10 @@
 #include <drm/radeon_drm.h>
 
 #include "radeon.h"
+#include "radeon_atombios.h"
 #include "radeon_legacy_encoders.h"
 #include "atom.h"
 
-extern void
-radeon_atom_backlight_init(struct radeon_encoder *radeon_encoder,
-			   struct drm_connector *drm_connector);
-
-
 static uint32_t radeon_encoder_clones(struct drm_encoder *encoder)
 {
 	struct drm_device *dev = encoder->dev;