msm: mdss: Correct set backlight call during unblank

If brightness node is written prior to unblank by surfaceflinger
the backlight level being applied would not be correct. The
stored value is also overwritten.

Passing the correct unset backlight value during unblank.

Change-Id: I430bc759e09ecc6dfbc616af47fbc953aa542b9a
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>