drm/amd/display: Add more tests for HDCP

drm/amd/display: Add more tests for HDCP

Adds a few more test cases to the hdcp code

  - process_output(): make sure work gets requeued when callback and
    watchdog are stopped but also asked to run again
  - event_property_update(): bail out when the connector is NULL
  - hdcp_handle_cpirq(): schedules the work and picks the right link
  - hdcp_update_display(): schedules property validation on enable,
    resets status and cancels validation on disable
  - hdcp_create_workqueue(): returns NULL when max links is zero

Assisted-by: Copilot:Claude-Opus-4.8
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Bhawanpreet Lakha <bhawanpreet.lakha@amd.com>
Signed-off-by: George Zhang <george.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>