drm/amd/display: Fix DCN2.1 default DSC clocks

[WHY]
Low dscclk in high vlevels blocks some DSC modes.

[HOW]
Update dscclk to 1/3 of dispclk.

Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>