drm/vc4: Hook up plane prepare_fb to lookup dma-buf reservations.

This way drm_atomic_helper_wait_for_fences() will actually do
something.  The vc4_seqno_cb has been doing the fence waits on V3D
manually, so far.
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170621185002.28563-1-eric@anholt.netAcked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>

drivers/gpu/drm/vc4/vc4_plane.c

@@ -759,9 +759,26 @@ void vc4_plane_async_set_fb(struct drm_plane *plane, struct drm_framebuffer *fb)
 	vc4_state->dlist[vc4_state->ptr0_offset] = addr;
 }
 
+static int vc4_prepare_fb(struct drm_plane *plane,
+			  struct drm_plane_state *state)
+{
+	struct vc4_bo *bo;
+	struct dma_fence *fence;
+
+	if ((plane->state->fb == state->fb) || !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->resv);
+	drm_atomic_set_fence_for_plane(state, fence);
+
+	return 0;
+}
+
 static const struct drm_plane_helper_funcs vc4_plane_helper_funcs = {
 	.atomic_check = vc4_plane_atomic_check,
 	.atomic_update = vc4_plane_atomic_update,
+	.prepare_fb = vc4_prepare_fb,
 };
 
 static void vc4_plane_destroy(struct drm_plane *plane)