msm: mdss:set brightness to min_bl if brightness is

less than min_bl.

For some backlight controllers,its minumul Duty is
10% (brightness 30), so we need to adjust the
brightness to min_bl when brightness is less than it.

Change-Id: I543764449074fdc0974f312fdbc4043f4f24d5c0
Signed-off-by: Shuo Yan <shuoy@codeaurora.org>