msm_fb: Fix race conditions for backlight update

Fix race conditions for backlight update thread to ensure
that panel specific artifacts are not observed during rapid
suspend/resume.

(cherry picked from commit cf6c96eea42169074ab477c7d189d628bad66090)

Change-Id: I82fedc2d9656fcce541f6fb9fef07b946ed3beb2
CRs-fixed: 424084
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>