msm: ext_display: update hpd and notify logic

Use "hpd" method to notify external display module about
cable status change and "notify" as an acknowledgment to
power on or off. This makes hpd method as a blocking
call completed by notify call.

Change-Id: I5ef7cf5c95d46a695f20a51214a2afabd6feb4b6
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>