drm/amd/display: Test top-level IRQ handler and vmin/vmax worker

drm/amd/display: Test top-level IRQ handler and vmin/vmax worker

[WHAT]
Add KUnit coverage for the two remaining uncovered functions in
amdgpu_dm_irq.c: amdgpu_dm_irq_handler() and dm_handle_vmin_vmax_update().

amdgpu_dm_irq_handler() is driven with a fake dc whose irq_service maps
the hardware IRQ entry to a DC source and whose per-source info table
lets dc_interrupt_ack() succeed; high- and low-context counting handlers
verify the immediate and scheduled dispatch paths.

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>