msm: mdss: Do not update backlight with max value

Calling backlight with u64_max value in display commit will set
unset_bl_level to max value and hence BL is not set with current
value during un-blank.

Change-Id: I52549b4b00f374b8f529a7841d56eb162bdbe511
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>