drm/amd/display: Test HPD interrupt handling

drm/amd/display: Test HPD interrupt handling

Add KUnit coverage for the HPD interrupt-handling helpers: the HPD-RX
offload worker, the HDMI HPD debounce worker, handle_hpd_irq_helper(),
handle_hpd_irq(), schedule_hpd_rx_offload_work() and handle_hpd_rx_irq().
Expose these statics for KUnit and add the stub link-service callbacks,
sink helpers and fixtures the tests rely on.

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