drm/msm/dp: trigger hpd callback when gpio hpd is initialized

Before gpio hpd depended on post_open to trigger hpd for the first
time. This change will trigger hpd internally to remove the dependency,
the behavior is also consistent with usbpd hpd.

Change-Id: Ic142c1af0e9037677d48efced123b40a651b77ce
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>