drm/vc4: Use the atomic state's commit workqueue.

Now that we're using the atomic helpers for fence waits, we can use
the same codepath as drm_atomic_helper_commit() does for async,
getting rid of our custom vc4_commit struct.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170621185002.28563-3-eric@anholt.net


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