drm/amd/display: Add KUnit test for PSR function

drm/amd/display: Add KUnit test for PSR function

Add KUnit tests for amdgpu_dm_psr_fill_caps() which
validates PSR capability population from DPCD data.

Export amdgpu_dm_psr_fill_caps() conditionally when
CONFIG_DRM_AMD_DC_KUNIT_TEST is enabled, following the
existing pattern used by CRC and HDCP test files.

The test covers PSR version mapping, RFB setup time
calculation, link training flag, DPCD field passthrough,
rate control caps, and power optimization flags.

Assisted-by: Copilot:Claude-Opus-4.6
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>