video: msm: Add backlight scaling for MDSS

Add backlight scaling functionality to the MDSS driver (includes the fixes
with regards atomicity of backlight scale and level updates). Allow for
power saving algorithms to modify the intensity of backlight without
introducing race conditions caused by competition for backlight level.

Change-Id: I97ae024055ca614648ffe2aeae09cc350d2577ec
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>