drm/amd/display: Test MST start/stop and panel settings

drm/amd/display: Test MST start/stop and panel settings

[WHAT]
Add KUnit coverage for dm_helpers_dp_mst_start_top_mgr and
dm_helpers_dp_mst_stop_top_mgr, dm_helpers_dp_write_hblank_reduction,
get_dsc_max_slices, dm_helpers_init_panel_settings,
dm_helpers_override_panel_settings and
fill_dc_mst_payload_table_from_drm.

Assisted-by: Copilot:Claude-Opus-4.8
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>