i965/gen6/gs: implement GS_OPCODE_SVB_WRITE opcode

This opcode will be used when sending SVB WRITE messages to save
transform feedback outputs into Streamed Vertex Buffers.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
4 files changed