vrend: fix failing GBM format lookup in vrend_renderer_pipe_resource_set_type()

vrend_renderer_pipe_resource_set_type() called for
VIRGL_RESOURCE_FD_DMABUF resources fail during virgl to GBM format
conversion due to an uninitialized variable.

Signed-off-by: Ryan Neph <ryanneph@google.com>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>

src/vrend_renderer.c

@@ -11051,6 +11051,7 @@ vrend_renderer_pipe_resource_set_type(struct vrend_context *ctx,
       if (!gr)
          return ENOMEM;
 
+      virgl_format = gr->base.format;
       drm_format = 0;
       if (virgl_gbm_convert_format(&virgl_format, &drm_format)) {
          vrend_printf("%s: unsupported format %d\n", __func__, virgl_format);