drm/amd/display: Add KUnit tests for amdgpu_dm_psr_set_event

drm/amd/display: Add KUnit tests for amdgpu_dm_psr_set_event

[Why & How]
Add three KUnit tests covering the early-exit validation guard in
amdgpu_dm_psr_set_event():

  - NULL stream argument returns false immediately
  - Valid stream with NULL link returns false
  - Valid stream/link with psr_feature_enabled == false returns false

Assisted-by: Copilot:Claude-Sonnet-4.6
Reviewed-by: Ray Wu <ray.wu@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>