winsys/radeon: sparse buffers will not be supported

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/winsys/radeon/drm/radeon_drm_bo.c

@@ -936,6 +936,8 @@ radeon_winsys_bo_create(struct radeon_winsys *rws,
     struct radeon_bo *bo;
     unsigned usage = 0, pb_cache_bucket;
 
+    assert(!(flags & RADEON_FLAG_SPARSE)); /* not supported */
+
     /* Only 32-bit sizes are supported. */
     if (size > UINT_MAX)
         return NULL;