msm: mdss: maintain AD state through idle power collapse

When device enters the idle power collapse mode, the entire
AD HW block is powered down, which will cause the previous
AD settings get lost. So when device exits the idle power
collapse mode, we need to store AD to previous state.

In order to achieve this, we need to switch AD mode to 0x85 first
and manually write the last AD strength value to AD register. Keep
this setting for couple of frames, in the meantime, kick off AD
calculator with t_filter_control value of 0 to ramp up the AD
strength from 0 to current target strength. Then switch the mode
back to auto strength (0x81).

Change-Id: Iddc12dbb0da06675141fa9fe049cfe90110defb9
Signed-off-by: Ping Li <pingli@codeaurora.org>