CHROMIUM: drm/exynos: fb: exynos_drm_fb functions take an exynos_drm_fb

CHROMIUM: drm/exynos: fb: exynos_drm_fb functions take an exynos_drm_fb

This is a simple refactoring that reduces some casting, and hopefully
reduces some of the driver's layer violations.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
BUG=chromium-os:37097
TEST=compile test

Change-Id: I2089215c2dc6fbbc6850126504114d1fe3b00401
Reviewed-on: https://gerrit.chromium.org/gerrit/39477
Tested-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Commit-Ready: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>