msm: mdss: Set backlight pwm level to 0 before disabling during BLANK

Currently, when BLANK is called to turn off the panel during suspend, we
set the backlight to level 0. The backlight pwm driver interprets this
as requiring pwm_disable() only. However, in order to prevent any
garbage data from being displayed on the panel during resume, we should
set the pwm level to 0 before disabling the pwm backlight node.

Change-Id: Ifc12c49cbfcac76a5ca07e8c5e16b0fd023e1eeb
CRs-Fixed: 711782
Signed-off-by: Benet Clark <benetc@codeaurora.org>