drm/rockchip: Get rid of some unnecessary code

Current code implements prepare_fb and cleanup_fb callbacks only to
grab/release fb references, which is already done by atomic framework
when creating/destryoing plane state. Let's remove these
unused bits.
Signed-off-by: Tomasz Figa <tfiga@chromium.org>

drivers/gpu/drm/rockchip/rockchip_drm_vop.c

@@ -637,22 +637,6 @@ static void vop_plane_destroy(struct drm_plane *plane)
 	drm_plane_cleanup(plane);
 }
 
-static int vop_plane_prepare_fb(struct drm_plane *plane,
-				struct drm_plane_state *new_state)
-{
-	if (plane->state->fb)
-		drm_framebuffer_reference(plane->state->fb);
-
-	return 0;
-}
-
-static void vop_plane_cleanup_fb(struct drm_plane *plane,
-				 struct drm_plane_state *old_state)
-{
-	if (old_state->fb)
-		drm_framebuffer_unreference(old_state->fb);
-}
-
 static int vop_plane_atomic_check(struct drm_plane *plane,
 			   struct drm_plane_state *state)
 {
@@ -845,8 +829,6 @@ static void vop_plane_atomic_update(struct drm_plane *plane,
 }
 
 static const struct drm_plane_helper_funcs plane_helper_funcs = {
-	.prepare_fb = vop_plane_prepare_fb,
-	.cleanup_fb = vop_plane_cleanup_fb,
 	.atomic_check = vop_plane_atomic_check,
 	.atomic_update = vop_plane_atomic_update,
 	.atomic_disable = vop_plane_atomic_disable,