drm: Use plane->state->fb over plane->fb

Stop looking at plane->fb on atomic drivers. Use plane->state->fb
instead.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180322152313.6561-9-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index 537ffaa..a231dd5 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -597,13 +597,20 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
 		/* If we have a mode we need a framebuffer. */
 		/* If we pass -1, set the mode with the currently bound fb */
 		if (crtc_req->fb_id == -1) {
-			if (!plane->fb) {
+			struct drm_framebuffer *old_fb;
+
+			if (plane->state)
+				old_fb = plane->state->fb;
+			else
+				old_fb = plane->fb;
+
+			if (!old_fb) {
 				DRM_DEBUG_KMS("CRTC doesn't have current FB\n");
 				ret = -EINVAL;
 				goto out;
 			}
 
-			fb = plane->fb;
+			fb = old_fb;
 			/* Make refcounting symmetric with the lookup path. */
 			drm_framebuffer_get(fb);
 		} else {