drm/msm/dp: check for sink count before enabling post_open callback

When the DP cable is disconnected at the sink side with the type-C cable
connected to the device, some dongles retain the hpd_high variable to
'true' but change the sink count to zero as part of the attention
callback. Check for sink count reported by the dongle in addition to
hpd_high status variable before enabling post_open callback during DP
disable sequence.

Change-Id: I002d039f79543eb6176df2eb2b6c648f586be429
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>