drm/vc4: Use drm_gem_fb_prepare_fb

vc4 has switched to using drm_fb->obj[], so we can just use the helper
unchanged.

v2: Make it compile ... oops.

Cc: Eric Anholt <eric@anholt.net>
Cc: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190625204208.5614-4-daniel.vetter@ffwll.ch

drivers/gpu/drm/vc4/vc4_plane.c

@@ -23,6 +23,7 @@
 #include <drm/drm_fb_cma_helper.h>
 #include <drm/drm_plane_helper.h>
 #include <drm/drm_atomic_uapi.h>
+#include <drm/drm_gem_framebuffer_helper.h>
 
 #include "uapi/drm/vc4_drm.h"
 #include "vc4_drv.h"
@@ -1126,16 +1127,14 @@ static int vc4_prepare_fb(struct drm_plane *plane,
 			  struct drm_plane_state *state)
 {
 	struct vc4_bo *bo;
-	struct dma_fence *fence;
 	int ret;
 
 	if (!state->fb)
 		return 0;
 
 	bo = to_vc4_bo(&drm_fb_cma_get_gem_obj(state->fb, 0)->base);
 
-	fence = reservation_object_get_excl_rcu(bo->base.base.resv);
-	drm_atomic_set_fence_for_plane(state, fence);
+	drm_gem_fb_prepare_fb(plane, state);
 
 	if (plane->state->fb == state->fb)
 		return 0;