msm: mdss: Add backlight attenuation mechanism in AD auto strength mode

AD backlight attenuation provides the ability to modulate backlight based
on calibration before feeding it to AD core for strength computation.

Change-Id: Iea011807b25882463b1142e17324fa0509007aa4
Signed-off-by: Ping Li <quicpingli@codeaurora.org>
msm: mdss: Move histogram completion initialization to pp_init

The histogram completion structure only needs to be initialized fully
once, and this can be done when the pp resource structure is created.
Otherwise, when enabling the histograms, it is sufficient to use the
reusable INIT_COMPLETION, in order to avoid any wait_queue errors.

Change-Id: Id4bfb19c91bd5ab07c01a5eb347ba50621fe8a97
Signed-off-by: Benet Clark <benetc@codeaurora.org>