drm: Don't try and disable an encoder that was never enabled

Prevents code that assumes that the encoder is active when asked to be
disabled from dying a horrible death.
Reported-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_crtc_helper.c

@@ -241,7 +241,7 @@ void drm_helper_disable_unused_functions(struct drm_device *dev)
 	}
 
 	list_for_each_entry(encoder, &dev->mode_config.encoder_list, head) {
-		if (!drm_helper_encoder_in_use(encoder)) {
+		if (encoder->crtc && !drm_helper_encoder_in_use(encoder)) {
 			drm_encoder_disable(encoder);
 			/* disconnector encoder from any connector */
 			encoder->crtc = NULL;