msm: mdss: fix backlight issue on blank-unblank

In charging mode if charging code is not updating
the backlight then backlight will be turned off in
blank call and will not be turned on again on unblank.
Same is seen during ESD recovery. Set the unset
backlight level to the current backlight level during
blank so that after unblank, on first commit the
backlight value is restored. Also initialize the
brightness level to half of the max brightness.

Change-Id: Iee29b00a5e0fbbd4fee9d2c0dae1a40222b43939
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>