drm/amd/display: Add KUnit tests for amdgpu_dm_psr

drm/amd/display: Add KUnit tests for amdgpu_dm_psr

[WHAT]
Add Kunit tests for functions:
- link_supports_psrsu()
- amdgpu_dm_psr_fill_caps()
- amdgpu_dm_set_psr_caps()
- amdgpu_dm_psr_is_active_allowed()
- amdgpu_dm_psr_set_event()

Assisted-by: Copilot:GPT-5.5
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>