drm/radeon/radeon_drv: Move 'radeon_gem_prime_import_sg_table()'s prototype to shared header

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

drivers/gpu/drm/radeon/radeon_prime.c:43:24: warning: no previous prototype for ‘radeon_gem_prime_import_sg_table’ [-Wmissing-prototypes]
43 | struct drm_gem_object *radeon_gem_prime_import_sg_table(struct drm_device *dev,
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Cc: Alex Deucher <[e-mail]>
Cc: "Christian König" <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Gareth Hughes <[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_drv.c

···
 #include "radeon_kms.h"
 #include "radeon_ttm.h"
 #include "radeon_device.h"
+#include "radeon_prime.h"
 
 /*
  * KMS wrapper.
···
 int radeon_mode_dumb_create(struct drm_file *file_priv,
 			    struct drm_device *dev,
 			    struct drm_mode_create_dumb *args);
-struct drm_gem_object *radeon_gem_prime_import_sg_table(struct drm_device *dev,
-							struct dma_buf_attachment *,
-							struct sg_table *sg);
 
 /* atpx handler */
 #if defined(CONFIG_VGA_SWITCHEROO)

drivers/gpu/drm/radeon/radeon_prime.h

···
 void radeon_gem_prime_unpin(struct drm_gem_object *obj);
 void *radeon_gem_prime_vmap(struct drm_gem_object *obj);
 void radeon_gem_prime_vunmap(struct drm_gem_object *obj, void *vaddr);
+struct drm_gem_object *radeon_gem_prime_import_sg_table(struct drm_device *dev,
+							struct dma_buf_attachment *,
+							struct sg_table *sg);
 
 #endif				/* __RADEON_PRIME_H__ */