drm/vc4: Fix VBLANK handling in crtc->enable() path

When we are enabling a CRTC, drm_crtc_vblank_get() is called before
drm_crtc_vblank_on(), which is not supposed to happen (hence the
WARN_ON() in the code). To solve the problem, we delay the 'update
display list' operation after the CRTC is actually enabled.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/1498163126-26678-1-git-send-email-boris.brezillon@free-electrons.com
Fixes: 34c8ea40 ("drm/vc4: Mimic drm_atomic_helper_commit() behavior")