drm/armada: clean up armada_drm_crtc_plane_disable()

Merge armada_drm_primary_disable() into armada_drm_crtc_plane_disable()
and rename to armada_drm_plane_disable().  Use this to simplify
armada_ovl_plane_disable().

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
diff --git a/drivers/gpu/drm/armada/armada_overlay.c b/drivers/gpu/drm/armada/armada_overlay.c
index cf84425..a53e7dd 100644
--- a/drivers/gpu/drm/armada/armada_overlay.c
+++ b/drivers/gpu/drm/armada/armada_overlay.c
@@ -264,15 +264,12 @@
 {
 	struct armada_ovl_plane *dplane = drm_to_armada_ovl_plane(plane);
 	struct drm_framebuffer *fb;
-	struct armada_crtc *dcrtc;
 
-	if (!dplane->base.base.crtc)
-		return 0;
+	armada_drm_plane_disable(plane, ctx);
 
-	dcrtc = drm_to_armada_crtc(dplane->base.base.crtc);
-	armada_drm_crtc_plane_disable(dcrtc, plane);
+	if (dplane->base.base.crtc)
+		drm_to_armada_crtc(dplane->base.base.crtc)->plane = NULL;
 
-	dcrtc->plane = NULL;
 	dplane->base.state.ctrl0 = 0;
 
 	fb = xchg(&dplane->old_fb, NULL);