radeon: add fallthrough annotation

GCC 7 started warning when a switch case has neither a `break` nor
a "fallthrough" comment.
Let's be explicit that we meant to fall through here.

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>

radeon/radeon_surface.c

@@ -2503,6 +2503,7 @@ static int radeon_surface_sanity(struct radeon_surface_manager *surf_man,
         if (surf->npix_y > 1) {
             return -EINVAL;
         }
+        /* fallthrough */
     case RADEON_SURF_TYPE_2D:
         if (surf->npix_z > 1) {
             return -EINVAL;