drm/radeon/kms: rework pll algo selection

Rework the pll algo selection so that the pll algo
in use can be selected more easily.  This allows
us to select different pll divider selection algos
for specific monitors that work better with one algo
or the other.  This is needed for the next patch which
adds an LVDS pll quirk for a specific notebook.

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