drm/radeon/kms/avivo: add support for new pll selection algo

Supported on all AVIVO-based asics.
Can be disabled via the new_pll module parameter:
new_pll=0 - disable
new_pll=1 - enable
enabled by default

[airlied: fixed to use do_div]
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>