drm/radeon: R300 AD only has one quad pipe.

Gleaned from the Mesa code.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=27355

 .

Signed-off-by: Michel Dänzer <daenzer@vmware.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>