msm_fb: mdss: Add support for Backlight control and panel gpios

Add code to support the WLED client interface to control
backlight levels using led_trigger mechanism.

Add support for msm gpio that maps to the lcd_enable line
for the panel. Add support for pmic gpio that is mapped
to the display reset line.

Change-Id: Ie89692f1c0a38d7878271d4a91122d61fbba8c1b
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>