msm: mdss: fix backlight update between unblank and kickoff

The backlight update between unblank and kickoff
call may result in garbage/snow screen on panel
based on panel ram initialized status.

For ex:
  -> Unblank the panel - black frame transfer start,
                         panel is on but ram is still
                         uninitialized.
  -> backlight update - turn on the backlight. It will show
                        data from uninitialized ram because
                        frame transfer is not complete yet.

This can be avoided by blocking the backlight update
request between unblank and first frame kickoff. This
gives guarantee that panel has valid data to display
when backlight was turned on.

Change-Id: I09c707cd10acd53bd6aa0935885269ffc5aec649
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>