r600g: use common scissor and viewport code

r600g: use common scissor and viewport code

It's the same as radeonsi. This adds guard band support to r600g.

Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Grigori Goronzy <greg@chown.ath.cx>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>