drm/radeon/kms: Enable new pll calculation for avivo+ asics

New algo is used for r5xx+ and legacy is used for
r1xx-r4xx, rv515.

I've tested on all relevant GPUs and monitors that I
have access to and have found no problems.

Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=26562
https://bugzilla.kernel.org/show_bug.cgi?id=26552
May fix:
https://bugs.freedesktop.org/show_bug.cgi?id=32556



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