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
Change-Id: I708513cfcec558c5d65f21f189dd52331fdf0f4f
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>