drm/amd/display: Fix null-deref on vega20 with xgmi

[Why]
After clkmgr rework it gets initialized after resource pool.
The clkmgr is used in resource pool init for xgmi path.
That causes driver crash on Vega20 with xgmi due to NULL deref.

[How]
Move xgmi compensation code to dce121_clk_mgr_construct()
That also allows to make dce121_clock_patch_xgmi_ss_info()
internal static function.

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>