drm/msm/dp: fix MST sink count and attention message handling

Update sink count when detected by panel as no downstream devices
attached. This avoids missing sink update interrupt from sink.
Also, fix the attention message not being routed to MST handling.

CRs-Fixed: 2316018
Change-Id: I5fb2c04a62ffeb4b6acf90f6d18e1a8ba7d0e329
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>
Signed-off-by: Divya Sharma <divyash@codeaurora.org>