msm: mdss: update backlight during unblank if required

Some panel don't require backlight to be updated during
unblank and only require update upon first commit. Add
support to update the backlight based on panel specific
property.

Change-Id: I43f33505be5151640ad7dc2ee1a14df8a55a6dfe
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>