drm/amd/display: Compensate for XGMI SS downspread on dprefclk

[Why]
When XGMI is enabled, we need to adjust the dprefclk according to the
WAFL link's spread spectrum info. This is for VG20 (DCE121) only.

[How]
dce_clk_mgr already stores SS info, currently being used by audio clock.
Therefore, patch the clk_mgr's SS info with the xGMI SS info, if xGMI
is enabled. For display clock, adjust it during dce12_update_clocks()
before calling set_clock().

Since we rely on a mmhub register to reliably determine if xGMI is
enabled, the patching step needs to happen after resource_construct()
has initialized the hardware sequencer.

Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>