msm: board-8930: add gpio-controlled backlight function

The function pointer is passed to driver to turn on/off backlight to
avoid garbage display.

(cherry picked from commit a07d78743024144158a7b2cc0c194ae75e9b6bea)

CRs-fixed: 408728
Change-Id: Ibb218246bd3414b7004289e5afd0594a186374f1
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>

Conflicts:
	arch/arm/mach-msm/board-8930-display.c
	arch/arm/mach-msm/include/mach/board.h