drm/msm/dp: fix handling of branch devices

A branch device usually notifies a change in downstream connections
using the HPD IRQ pulse. Handle this by checking for a change in
downstream sink count and appropriately handling EDID reads. It is
also possible that the branch device may not have any local EDID.
In such cases, when the downstream sink count is zero, do not read
EDID.

CRs-Fixed: 2093830
Change-Id: I230560c995d7c3b395e37aef5483e5468e1d1dec
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>