msm_fb: Increase PWM granularity to 255 levels for 8960 fluid

Currently 8960 fluid display panel only supports 100 levels for backlight.
Content Adaptive Backlight Control requires 255 levels of backlight for
optimal user experience with CABL turned on.

CRs-fixed: 342485

Change-Id: I02226a7790f2dd0144c1eb623c8177724e33a715
Signed-off-by: Pravin Tamkhane <pravint@codeaurora.org>