r600g: setup COLOR1 for possible dual-src in the framebuffer bind

r600g: setup COLOR1 for possible dual-src in the framebuffer bind

As pointed out by Marek, if we have only one cb, we may as well add this
single register write here rather than adding it in the draw loop.

Reviewed-by: Marek Olšák <maraeo@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>