msm: mdss: use platform-enable-gpio on msm8909w BG

Use platform-enable-gpio to control buffer connected
on panel reset gpio.
This helps in avoiding panel reset during TWM entry.

Change-Id: I6a0706c593202ab62acf332690d3255acbfd49f8
Signed-off-by: Sachin Bhayare <sachin.bhayare@codeaurora.org>