drm/msm/dp: enable gpio aux switch support

If we defined the gpios for a gpio DP AUX switch
chipset, it was ignored by the default settings.
We should enable it as same as I2C chipset and HPD
gpio detected methods.

Signed-off-by: Yuan Zhao <yzhao@codeaurora.org>
Change-Id: Ife110776a79808d9de962ad93fae7b46ea79658a