drm/radeon: don't use fractional dividers on RS[78]80 if SS is enabled

Seems to cause problems for some older hardware. Kudos to Thom Kouwenhoven
for working a lot with the PLLs and figuring this out.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>