drm/amd/display: Add KUnit test for ISM functions

drm/amd/display: Add KUnit test for ISM functions

Add KUnit tests for three static functions in amdgpu_dm_ism.c:
dm_ism_next_state, dm_ism_get_sso_delay, and
dm_ism_get_idle_allow_delay.

The 32 test cases cover the full FSM transition table,
SSO delay calculation with various timings, and
hysteresis-based idle allow delay including circular
buffer wraparound and old history cutoff logic.

Conditionally remove static linkage and export the three
functions under CONFIG_DRM_AMD_DC_KUNIT_TEST so the test
module can call them.

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>