drm/radeon/kms: rework spread spectrum handling

This patch reworks spread spectrum handling to enable it
properly on lvds and DP/eDP links.  It also fixes several
bugs in the old spread spectrum code.

- Use the ss recommended reference divider if available
when calculating the pll
- Use the proper ss command tables on pre-DCE3 asics
- Avoid reading past the end of the ss info tables
- Enable ss on evergreen asics (lvds, dp, tmds)
- Enable ss on DP/eDP links

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