msm: mdss: lock mutex before setting backlight

Backlight set function call should be protected by mutex
as this call could be called from different threads.

CRs-Fixed: 1074738
Change-Id: Ie7f8cee59b90f16f8a844d618a6f903b3e3c2f27
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>