drm: Add helpers to turn off CRTCs

Turning off a single CRTC or all active CRTCs of a DRM device is a
fairly common pattern. Add helpers to avoid open coding this everywhere.

The name was chosen to be consistent with drm_plane_force_disable().

Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index f5469d3..781695c 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -2654,6 +2654,8 @@
 extern void drm_plane_force_disable(struct drm_plane *plane);
 extern void drm_crtc_get_hv_timing(const struct drm_display_mode *mode,
 				   int *hdisplay, int *vdisplay);
+extern int drm_crtc_force_disable(struct drm_crtc *crtc);
+extern int drm_crtc_force_disable_all(struct drm_device *dev);
 
 extern void drm_encoder_cleanup(struct drm_encoder *encoder);