drm/kms: add crtc disable function

More explicit than dpms. Same as the encoder disable function.

Need this to explicity disconnect plls from crtcs for reuse when you
plls:crtcs ratio isn't 1:1.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/drm_crtc_helper.c b/drivers/gpu/drm/drm_crtc_helper.c
index 9b2a541..fa1323f 100644
--- a/drivers/gpu/drm/drm_crtc_helper.c
+++ b/drivers/gpu/drm/drm_crtc_helper.c
@@ -241,7 +241,10 @@
 		struct drm_crtc_helper_funcs *crtc_funcs = crtc->helper_private;
 		crtc->enabled = drm_helper_crtc_in_use(crtc);
 		if (!crtc->enabled) {
-			crtc_funcs->dpms(crtc, DRM_MODE_DPMS_OFF);
+			if (crtc_funcs->disable)
+				(*crtc_funcs->disable)(crtc);
+			else
+				(*crtc_funcs->dpms)(crtc, DRM_MODE_DPMS_OFF);
 			crtc->fb = NULL;
 		}
 	}