drm/msm/dp: handle hdcp 2.2 polling in link work

When polling for a message from sink during hdcp 2.2 authentication
process, handle the sink response in link worker thread only. The
link worker thread is triggered by CP_IRQ set by the sink. This avoids
possible race condition of handling the sink request which may result
in reading same message twice from sink.

CRs-Fixed: 2371750
Change-Id: If3ee31ec84d3f47195bcba6917c0b76ec89ae535
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>