drm/radeon/kms/atom: spread spectrum fix

The atom spread spectrum table does not always
disable ss.  Explicitly disable it and then use
the atom table to enable later if needed (currently
only used for LVDS).

Fixes display issues on some systems.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>