drm/amd/display: Add KUnit test for HDCP process_output

drm/amd/display: Add KUnit test for HDCP process_output

Expose process_output() as non-static when CONFIG_DRM_AMD_DC_KUNIT_TEST
is enabled and add KUnit tests exercising its full branch logic:

- property_validate_dwork is always enqueued (delay=0)
- callback_dwork is scheduled when callback_needed is set
- callback_dwork is cancelled when callback_stop is set
- watchdog_timer_dwork is scheduled when watchdog_timer_needed is set
- watchdog_timer_dwork is cancelled when watchdog_timer_stop is set
- Both dworks are scheduled independently when both flags are set

Assisted-by: Copilot:Claude-Sonnet-4.6
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>