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

 drivers/gpu/drm/radeon/radeon_prime.c:34:18: warning: no previous prototype for ‘radeon_gem_prime_get_sg_table’ [-Wmissing-prototypes]
 34 | struct sg_table *radeon_gem_prime_get_sg_table(struct drm_gem_object *obj)
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/radeon_prime.c:42:7: warning: no previous prototype for ‘radeon_gem_prime_vmap’ [-Wmissing-prototypes]
 42 | void *radeon_gem_prime_vmap(struct drm_gem_object *obj)
 | ^~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/radeon_prime.c:55:6: warning: no previous prototype for ‘radeon_gem_prime_vunmap’ [-Wmissing-prototypes]
 55 | void radeon_gem_prime_vunmap(struct drm_gem_object *obj, void *vaddr)
 | ^~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/radeon_prime.c:62:24: warning: no previous prototype for ‘radeon_gem_prime_import_sg_table’ [-Wmissing-prototypes]
 62 | struct drm_gem_object *radeon_gem_prime_import_sg_table(struct drm_device *dev,
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/radeon_prime.c:86:5: warning: no previous prototype for ‘radeon_gem_prime_pin’ [-Wmissing-prototypes]
 86 | int radeon_gem_prime_pin(struct drm_gem_object *obj)
 | ^~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/radeon_prime.c:104:6: warning: no previous prototype for ‘radeon_gem_prime_unpin’ [-Wmissing-prototypes]
 104 | void radeon_gem_prime_unpin(struct drm_gem_object *obj)
 | ^~~~~~~~~~~~~~~~~~~~~~
 drivers/gpu/drm/radeon/radeon_prime.c:120:17: warning: no previous prototype for ‘radeon_gem_prime_export’ [-Wmissing-prototypes]
 120 | struct dma_buf *radeon_gem_prime_export(struct drm_gem_object *gobj,
 | ^~~~~~~~~~~~~~~~~~~~~~~

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>