msm: mdss: Minimize cases of BL lock in update backlight

Calling backlight lock on every commit iteration was adding
overhead. Altering the conditions to allow locking only for
case when unset backlight level is not zero.

Change-Id: I023dccfa584cf641d042565b7ad2cb32c0724c55
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>