Revert "drm/radeon: rework pll selection (v3)"

This reverts commit 985f61f7.

This commit fixed certain cases, but ended up regressing others
due to limitations in the current KMS API.  A proper fix is too
invasive for 3.6.  Push it back to 3.7.

Reported-by: Andres Freund <andres@anarazel.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>