drm/amd/display: Test high-IRQ and handler registration

drm/amd/display: Test high-IRQ and handler registration

[WHAT]
Add the final batch of amdgpu_dm_irq KUnit tests covering the high-IRQ
callbacks (dm_pflip_high_irq, dm_vupdate_high_irq, dm_crtc_high_irq),
dm_handle_hpd_work, the DMUB outbox1 low IRQ, and the per-ASIC IRQ
handler registration helpers (DCE110, DCN10 and outbox).

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>