drm/gud: fix NULL fb and crtc dereferences on USB disconnect

On disconnect drm_atomic_helper_disable_all() is called which
sets both the fb and crtc for a plane to NULL before invoking a commit.

This causes a kernel oops on every display disconnect.

Add guards for those dereferences.

Cc: <[e-mail]> # 6.18.x
Fixes: 73cfd166e045 ("drm/gud: Replace simple display pipe with DRM atomic helpers")
Signed-off-by: Shenghao Yang <[e-mail]>
Reviewed-by: Ruben Wauters <[e-mail]>
Signed-off-by: Ruben Wauters <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

drivers/gpu/drm/gud/gud_pipe.c

···
 	struct drm_plane_state *old_plane_state = drm_atomic_get_old_plane_state(state, plane);
 	struct drm_plane_state *new_plane_state = drm_atomic_get_new_plane_state(state, plane);
 	struct drm_crtc *crtc = new_plane_state->crtc;
-	struct drm_crtc_state *crtc_state;
+	struct drm_crtc_state *crtc_state = NULL;
 	const struct drm_display_mode *mode;
 	struct drm_framebuffer *old_fb = old_plane_state->fb;
 	struct drm_connector_state *connector_state = NULL;
 	struct drm_framebuffer *fb = new_plane_state->fb;
-	const struct drm_format_info *format = fb->format;
+	const struct drm_format_info *format;
 	struct drm_connector *connector;
 	unsigned int i, num_properties;
 	struct gud_state_req *req;
 	int idx, ret;
 	size_t len;
 
-	if (drm_WARN_ON_ONCE(plane->dev, !fb))
-		return -EINVAL;
-
-	if (drm_WARN_ON_ONCE(plane->dev, !crtc))
-		return -EINVAL;
-
-	crtc_state = drm_atomic_get_new_crtc_state(state, crtc);
-
-	mode = &crtc_state->mode;
+	if (crtc)
+		crtc_state = drm_atomic_get_new_crtc_state(state, crtc);
 
 	ret = drm_atomic_helper_check_plane_state(new_plane_state, crtc_state,
 						  DRM_PLANE_NO_SCALING,
···
 
 	if (old_plane_state->rotation != new_plane_state->rotation)
 		crtc_state->mode_changed = true;
+
+	mode = &crtc_state->mode;
+	format = fb->format;
 
 	if (old_fb && old_fb->format != format)
 		crtc_state->mode_changed = true;
···
 	struct drm_atomic_helper_damage_iter iter;
 	int ret, idx;
 
-	if (crtc->state->mode_changed || !crtc->state->enable) {
+	if (!crtc || crtc->state->mode_changed || !crtc->state->enable) {
 		cancel_work_sync(&gdrm->work);
 		mutex_lock(&gdrm->damage_lock);
 		if (gdrm->fb) {