drm/radeon/kms: update new pll algo

- add support for pre-avivo chips
- add support for fixed post/ref dividers
- add support for non-fractional fb dividers

By default avivo chips use the new algo and
pre-avivo chips use the old algo. Use the "new_pll"
module option to toggle between them.

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