ARM: dts: msm: update backlight control for 8909 with PM8916

On regular 8909 device with PM8909 MPP2 is used for
PWM_WLED whereas the device with PM8916 needs to use
MPP4 for PWM_WLED. Update this in the dtsi file for
backlight control.

Change-Id: I8603e4d4858fc8c75bef15b0497a70bc99196896
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>