drm/ast: Add an crtc_disable callback to the crtc helper funcs

Implement the proper CRTC disablement, just like done in mgag200
driver.

Signed-off-by: Egbert Eich <eich@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/ast/ast_mode.c

@@ -613,7 +613,23 @@ static int ast_crtc_mode_set(struct drm_crtc *crtc,
 
 static void ast_crtc_disable(struct drm_crtc *crtc)
 {
+	int ret;
+
+	DRM_DEBUG_KMS("\n");
+	ast_crtc_dpms(crtc, DRM_MODE_DPMS_OFF);
+	if (crtc->primary->fb) {
+		struct ast_framebuffer *ast_fb = to_ast_framebuffer(crtc->primary->fb);
+		struct drm_gem_object *obj = ast_fb->obj;
+		struct ast_bo *bo = gem_to_ast_bo(obj);
+
+		ret = ast_bo_reserve(bo, false);
+		if (ret)
+			return;
 
+		ast_bo_push_sysram(bo);
+		ast_bo_unreserve(bo);
+	}
+	crtc->primary->fb = NULL;
 }
 
 static void ast_crtc_prepare(struct drm_crtc *crtc)