msm_fb: Update backlight level on resume after first pan display

Use shared workqueue to delay the update of backlight after
a first valid frame is displayed on the panel. This avoids any
transients being displayed during resume.

CRs-fixed: 361002
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>
(cherry picked from commit 8c13d2615b1c608f941137166efb649353cc0bdb)

Signed-off-by: Gopal Goberu <ggober@codeaurora.org>
(cherry picked from commit 8a17f54f8d9e267b03b41df59bf210d56e16064a)

Change-Id: I641738dbe90bb3fb162d1c07d19ef587c071dd61
Signed-off-by: Gopal Goberu <ggober@codeaurora.org>