r600g: handle the write all cbufs property.

r600g: handle the write all cbufs property.

This only works on r600/r700 so far, evergreen doesn't appear
to have the multiwrite enable bit in the color control, so we
may have to actually do a shader rewrite on EG hardware.

remove some duplicate code reg defines also.

Signed-off-by: Dave Airlie <airlied@redhat.com>