msm_fb: display: add setting gpio-controlled backlight logic

For the panels which backlight is controlled by gpio, add this logic
in driver to turn on/off backlight, instead of calling board panel
on/off functions, to avoid garbage display due to early backlight
turn-on.

CRs-fixed: 408728
Change-Id: I6f1b9bd78d96dbf1492e854edd59b05f8dec205f
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>