disp: msm: dp: skip attention requests on disconnect

Don't process any attention requests after hpd off. During automation
tests, scripts are doing a mst unplug after hpd off. But sometimes
disconnect thread freezes waiting for audio ack. During this freeze
unplug happens and uninstalls connector and free the panel and audio
objects due to which lock associated with completion variable causes an
exception.

Change-Id: I4038fd7debc383a96d60906eef17ec6e2163b326
Signed-off-by: Rajat Gupta <rajatgu@codeaurora.org>