disp: msm: dsi: add a new PWM pinctrl configure

The default PWM gpio pin mode was not right for PWM backlight
control. If needs to use PWM backlight, must config the
gpio pinctrl.

Change-Id: I5fd6b947d379b53ef4c358be1b935a2ad4970f99
Signed-off-by: Yuan Zhao <yzhao@codeaurora.org>