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>
diff --git a/radeon/radeon_surface.c b/radeon/radeon_surface.c
index 965be24..04df77d 100644
--- a/radeon/radeon_surface.c
+++ b/radeon/radeon_surface.c
@@ -2503,6 +2503,7 @@
         if (surf->npix_y > 1) {
             return -EINVAL;
         }
+        /* fallthrough */
     case RADEON_SURF_TYPE_2D:
         if (surf->npix_z > 1) {
             return -EINVAL;