drm/exynos: remove NULL check from exynos_drm_overlay_update

drm/exynos: remove NULL check from exynos_drm_overlay_update

We really shouldn't be handling this NULL pointer. If there
is a NULL pointer here, we've hit a kernel bug. No need for a
BUG_ON since the kernel will panic on any NULL dereference.

This allows us to make exynos_drm_overlay_update void.

BUG=chrome-os-partner:15349,chrome-os-partner:14965
TEST=Multiple VT switch, HDMI plug/unplug, sign in/out, suspend/resume,
     idle suspend/resume.

Change-Id: I39c4ee44260eaeb98594dfbe01f2b885039ab23c
Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/35248
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>