drm/amd/display: Cover crtc set_vblank workqueue branch

Add dm_test_crtc_enable_vblank_queues_work and
dm_test_crtc_disable_vblank_queues_work to cover the
vblank_control_workqueue branch of amdgpu_dm_crtc_set_vblank():

- The enable test installs a real workqueue, retains the stream and
  queues the control worker, then drains it and checks the active vblank
  IRQ count was incremented.
- The disable test drives the no-stream sub-branch (the stream-retain is
  skipped) and checks the worker decremented the count.

Both seed the ISM so the queued worker takes no state-machine transition,
keeping coverage on the vblank accounting; the ISM state machine itself
is covered by the ISM tests.

Assisted-by: Copilot:Claude-Opus-4.8
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Bhawanpreet Lakha <bhawanpreet.lakha@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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