drm/radeon/kms: don't default display priority to high on rs4xx

Seems to cause issues with the sound hardware.  Fixes kernel
bug 15982:
https://bugzilla.kernel.org/show_bug.cgi?id=15982



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