drm/msm/sde: add connector support for bl scale

Create backlight scale properties for AD and ABA in connector,
and scale backlight when set DSI backlight level.

Change-Id: I2d1b157d63938c1c1fc6e396b37b142edf6a1e22
Signed-off-by: Xu Yang <yangxu@codeaurora.org>
Signed-off-by: Ping Li <pingli@codeaurora.org>