CHROMIUM: drm/exynos: get/put the fb for every page flip

CHROMIUM: drm/exynos: get/put the fb for every page flip

We had been using kds to prevent unmapping the fb while its
under scanout. But not all fbs are KDS-backed nor is it a
good idea to assume user-space is exporting all fbs.

BUG=chrome-os-partner:15349,chrome-os-partner:14965
TEST=Multiple VT switch, sign in/out.

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Change-Id: I33958f65f961d328db75c32c5597edd5666fb708
Reviewed-on: https://gerrit.chromium.org/gerrit/38897
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>