drm/amd/display: dcn2 dmcu wait_for_loop update with dispclk.

[Description]
DMUB is using DPREF CLK, but DMCU still use displayclk.
This is for updating DMCU wait_for_loop after display clock change.

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>