drm/radeon/kms: switch back to min->max pll post divider iteration

Seems more reliable.  Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=26552



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