drm/msm/dp: Add gpio hpd support for DP

For board with direct DP output, there is no usbpd
controller or aux switch. The change will add gpio
based hpd to support direct DP connection.

A super interface dp_hpd is added on top of dp_usbpd
to abstrct the hpd functions. With the change, usbpd
and gpio_hpd are two instances of dp_hpd.

CRs-Fixed: 2262783
Change-Id: I798f52445d0e64bd8c0f7a90f4e9815811b047fc
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>