msm: mdss: add backlight gpio invert support for display

Some platforms may use external gpio to enable and disable backlight,
and we may need to invert this gpio according to HW design,
so add support for that.

CRs-Fixed: 1109294
Change-Id: Ib5e895eebcc38d185e8b703c3d895781b43c58c7
Signed-off-by: Yahui Wang <yahuiw@codeaurora.org>