drm/radeon/kms: Adjust pll picker for DCE6.1

On TN, UNIPHYA always uses PPLL2, UNIPHYB/C/D/E/F
can use either PPLL1 or PPLL0.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>