msm: board-8064: Add backlight control when using PM8917

Use PM8917 GPIO 26 to enable and disable the LCD backlight on
targets which use PM8917 since PWM control is not available.

Change-Id: Ic0a2b840e41e3c4ec90af3cbee2d3f494623bc63
Signed-off-by: David Collins <collinsd@codeaurora.org>