drm/vc4: Mimic drm_atomic_helper_commit() behavior

The VC4 KMS driver is implementing its own ->atomic_commit() but there
are a few generic helpers we can use instead of open-coding the logic.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/1496392332-8722-4-git-send-email-boris.brezillon@free-electrons.com