r600g: rework scissor for r600-r700

r600g: rework scissor for r600-r700

We must use VPORT_SCISSOR, because that's the only one we can use for multiple
scissor rectangles in ARB_viewport_array.

R700 can use the VPORT_SCISSOR_ENABLE bit, but R600 doesn't have that and must
emit a 8192x8192 rectangle if scissor is disabled.

This commit also cleanups magic numbers in create_rs_state.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>