drm/exynos: Check for null dev_priv and return

drm/exynos: Check for null dev_priv and return

During suspend resume we seem to be getting a NULL dev_priv
Good to check and return when null.

BUG=none
TEST=Crash is hard to reproduce tried 10 times. But hopefully
wont happen

Change-Id: I2c99e86103bc23f7b62f417e48232c98d6eca176
Signed-off-by: Anush Elangovan <anush@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/28568
Reviewed-by: Pawel Osciak <posciak@google.com>
Commit-Ready: Anush Elangovan <anush@google.com>
Tested-by: Anush Elangovan <anush@google.com>