msm: ext_display: update hpd and notify for HDMI

The semantic of hpd and notify have been changed. hpd is
a blocking call which notifies display and audio with the
cable status, and notify is used to complete the hpd call
which waits for the acknowledgment of power on/off. Update
these 2 interface implementations for HDMI.

CRs-Fixed: 1086894
Change-Id: I8603149132e224cb2859d2b808123238d0516817
Signed-off-by: Ray Zhang <rayz@codeaurora.org>
Signed-off-by: Lei Chen <chenl@codeaurora.org>