drm/dp: defer h/w hpd registeration to the end of dp sub module init

Previously when hpd_get is called, it will register callback
function into hardware. There is a potential racing issue that hardware
may call the callback function immediately in a separate thread
when the rest dp sub module initializations are not finished yet.

A new api register_hpd is added to each dp_hpd type and will get
called when all sub modules are initialized. H/W registeration will
be moved to this new api to guarantee dp is fully initialized before
any callback would happen.

CRs-Fixed: 2359246
Change-Id: I941a5c11e09ad05cc336c307940cba7749ff84e8
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>