drm/xen-front: use simple display pipe prepare_fb helper

I missed this one because on an older tree.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Cc: xen-devel@lists.xen.org
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180409085134.27321-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/xen/xen_drm_front_kms.c

@@ -226,12 +226,6 @@ static bool display_send_page_flip(struct drm_simple_display_pipe *pipe,
 	return false;
 }
 
-static int display_prepare_fb(struct drm_simple_display_pipe *pipe,
-			      struct drm_plane_state *plane_state)
-{
-	return drm_gem_fb_prepare_fb(&pipe->plane, plane_state);
-}
-
 static void display_update(struct drm_simple_display_pipe *pipe,
 			   struct drm_plane_state *old_plane_state)
 {
@@ -294,7 +288,7 @@ static const struct drm_simple_display_pipe_funcs display_funcs = {
 	.mode_valid = display_mode_valid,
 	.enable = display_enable,
 	.disable = display_disable,
-	.prepare_fb = display_prepare_fb,
+	.prepare_fb = drm_gem_fb_simple_display_pipe_prepare_fb,
 	.update = display_update,
 };