drm/amd/display: Initialize HDCP work queue

[Why]
We need this to enable HDCP on linux, as we need events to interact
with the hdcp module

[How]
Add work queue to display manager and handle the creation and destruction
of the queue

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>