drm/radeon/kms/atom: add support for spread spectrum (v2)

Spread spectrum is a periodic disturbance added
to the feedback divider to change the pixel clock
periodically to reduce interference.

Only enabled on LVDS.

v2: add support for r4xx and fix DCE 3

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