drm/radeon/si_dpm: Limit clocks on HD86xx part

Limit clocks on a specific HD86xx part to avoid
crashes (while awaiting an appropriate PP fix).

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org