fbdev: msm: Add backlight class support for FB driver

Update WLED device control Change from led to backlight class
support for fb driver which provides sysfs interface to userspace
to control display brightness.

Change-Id: I699130eb91dd47260fa8d4d9e21faf5a1fd0fc14
Signed-off-by: Rahul Shahare <rshaha@codeaurora.org>