drm/radeon/dpm: disable cac setup on SI

Disable cac setup on SI for now since it causes
strange performance level restrictions on certain
cards.  I suspect there may be issues with some of
the 64 bit fixed point double emulation that is used
to set up those parameters.  I need to double check
the math before this can be re-enabled.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>