gl-renderer: Take direct-display into consideration

The buffer_init function added with commit 83b37c0a, "renderers: pull
dmabuf initial setup out of attach", doesn't take into consideration the
the buffer's direct-display property.

Previously, gl_renderer_attach_dmabuf, wasn't being called when dmabuf's
direct-display  was turned on, but with commit 83b37c0a this has been changed.

So with commit 83b37c0a, linux_dmabuf_buffer_get_user_data will never
return a valid gb (gl buffer state), causing a crash using
direct-display extension. This adds an explicit check to return early
when this happens.

Fixes: 83b37c0a, "renderers: pull dmabuf initial setup out of attach"
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>