drm/radeon/kms: pll tweaks for r7xx

Prefer min m to max p only on pre-r7xx asics.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=36197

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