msm: mdss: Prevent backlight update during continuous splash

Store backlight level during continuous splash transition and
update the same in first commit. This ensures the backlight
is restored. Change prevents IOMMU page faults seen during
continuous splash due to backlight update commands sent during
continuous splash.

Change-Id: Ib09d90d7cf8020fcceb5f04d1750f59fd648be3d
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>