drm/msm/dp: process sink request on local thread

Do not check for sink test requests on USB thread. Check for
sink request can take some time which involves AUX transactions.
In case the cable is removed, AUX transactions will start failing.
This blocks the USB to report cable disconnection and DP will
be stuck in AUX transaction retry loop.
Execute the check for sink test requests in a separate thread
so that USB thread is unblocked and disconnect event can be
processed.

CRs-Fixed: 2316018
Change-Id: I219ad0de7730723bd134f96d3e221b8bc3e621d3
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>