drivers: gpu: Mark function as static in ttm_bo.c

Mark function as static because it is not used outside file
drm/ttm/ttm_bo.c.

This eliminates the following warning in drm/ttm/ttm_bo.c:
drivers/gpu/drm/ttm/ttm_bo.c:960:5: warning: no previous prototype for ‘ttm_bo_move_buffer’ [-Wmissing-prototypes]
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>

drivers/gpu/drm/ttm/ttm_bo.c

@@ -957,7 +957,7 @@ int ttm_bo_mem_space(struct ttm_buffer_object *bo,
 }
 EXPORT_SYMBOL(ttm_bo_mem_space);
 
-int ttm_bo_move_buffer(struct ttm_buffer_object *bo,
+static int ttm_bo_move_buffer(struct ttm_buffer_object *bo,
 			struct ttm_placement *placement,
 			bool interruptible,
 			bool no_wait_gpu)