drm/exynos: mixer: Move layer update out of vsync

drm/exynos: mixer: Move layer update out of vsync

Move the mixer's layer update command out of the vsync interrupt and do
it only once per layer per vsync. We also don't want to finish a
pageflip if a layer update is in progress since it will still be
scanning out and X/userspace will start drawing to the front buffer.

BUG=chrome-os-partner:13692
TEST=Tested on snow, no corruption detected

Change-Id: I811da726daaf52117f9a42a299dbc98b43d7c3bd
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/33252
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>