sna/dri2: Flush rendering to a reused front buffer before reporting

When the client calls DRI2GetBuffers it expects the buffer to be
coherent, that is all X rendering to it flushed to hardware for it to be
able to read back.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/sna/sna_dri2.c

@@ -643,6 +643,7 @@ sna_dri2_create_buffer(DrawablePtr draw,
 			assert(private->bo->pitch == buffer->pitch);
 			assert(private->bo->active_scanout);
 
+			kgem_bo_submit(&sna->kgem, private->bo);
 			private->refcnt++;
 			return buffer;
 		}