drm/sun4i: dsi: Avoid hotplug race with DRM driver bind

We need to make sure that the DRM driver is fully registered before
allowing the panel to be attached. Otherwise, we may trigger a hotplug
event before sun4i_framebuffer_init() sets up drm->mode_config.funcs,
causing a NULL pointer dereference.

Fixes: 1a2703bd ("drm/sun4i: dsi: Allow binding the host without a panel")
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20200217060906.15152-1-samuel@sholland.org

drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c

@@ -970,7 +970,7 @@ static int sun6i_dsi_attach(struct mipi_dsi_host *host,
 
 	if (IS_ERR(panel))
 		return PTR_ERR(panel);
-	if (!dsi->drm)
+	if (!dsi->drm || !dsi->drm->registered)
 		return -EPROBE_DEFER;
 
 	dsi->panel = panel;