msm: mdss: Use primary display backlight in WB AD case

When AD is configured to run on the writeback display, it needs to use the
primary display's backlight configuration for getting, setting, and being
alerted to backlight changes since the writeback display does not have a
backlight to itself.

Change-Id: I615ce878a948798c1ce0c49c427e361c73798335
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>
Signed-off-by: Ping Li <quicpingli@codeaurora.org>