msm: mdss: Handle backlight level 0 at bootup

Current implementation does not allow backlight value of zero to
be applied at update backlight function called at commit.
unset_bl_level variable uses 0 value for comparision to exit at
update backlight function and hence cannot be used to set zero level.
This variable is now changed to use uint32 max value other than
zero value for comparision to take early exit.

Change-Id: If257fefed14455624edf6daaee933712df545117
Signed-off-by: Raviteja Tamatam <travitej@codeaurora.org>