drm/radeon: only apply the SS fractional workaround to RS[78]80

Looks like some RV6xx have problems with that.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=97099



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>
Cc: stable@vger.kernel.org