msm: mdss: set brightness to min_bl if brightness is less than min_bl.

Some backlight controllers specify a minimum duty cycle for the
backlight brightness. If the brightness is less than it, the
controller can malfunction.

Change-Id: Ib0aadb207e224684d7bcbb9c346a909f10e0d6ae
Signed-off-by: Shuo Yan <shuoy@codeaurora.org>