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.net


Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>