drm/amd/display: Test HPD init/fini

drm/amd/display: Test HPD init/fini

Add KUnit coverage for amdgpu_dm_hpd_init() and amdgpu_dm_hpd_fini():
empty connector list, the per-connector HW fallback path, the analog
polling enable/disable path, and the base-driver irq-ref path. Export
both functions for KUnit and add shared drm_connector test helpers.

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>