msm: mdss: Fix check for backlight update during continuous splash

If the cont_splash_enabled added with "&&" for backlight update
check then the unset_bl_level will be set 0 in all cases. Fix this
to add "||" as condition check instead of "&&".

Change-Id: I9365bd9b9beaada78b49e9e79046715f05ea5b72
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>