Make applying gamma less laggy.

Applying gamma takes a while, so optimize out multiple set requests
occuring quickly after each other. Those appear e.g. when dragging the
sliders quickly.

Change-Id: I48c71b8e041679bcce932f99d198f4e6a027010a
merge CyanogenMod

Change-Id: I3248da15b7d46e0921952e6a11110e845297436d