drm/amd/display: fix access hdcp_workqueue assert

commit 3cf7cd3f upstream.

[Why] hdcp are enabled for asics from raven. for old asics
which hdcp are not enabled, hdcp_workqueue are null. some
access to hdcp work queue are not guarded with pointer check.

[How] add hdcp_workqueue pointer check before access workqueue.

Fixes: 82986fd6 ("drm/amd/display: save restore hdcp state when display is unplugged from mst hub")
Link: drm/amd#2444


Reported-by: Niklāvs Koļesņikovs <89q1r14hd@relay.firefox.com>
Reviewed-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>