video: msm_fb: Ensure backlight is scaled atomically.

Ensure that when backlight scaling ratio or backlight level is being
changed that the operation execute atomically. This prevents a backlight
level change from being overwritten when it interrupts a backlight scale
update.

CRs-Fixed: 404017
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>
(cherry picked from commit ce328861aa1f4fa489e9abfc9d1e02b13af2ba47)

Signed-off-by: Dhivya Subramanian <dthiru@codeaurora.org>
(cherry picked from commit 0584596f75f8f2b184b8be1a0479ee61e449acba)

Change-Id: Ifb4bd5e823815b51dae38f1b8647b80d21b8ee45
Signed-off-by: Sudhir Sharma <sudsha@codeaurora.org>