i965/gen6/gs: Skeleton for user GS program support

i965/gen6/gs: Skeleton for user GS program support

Currently, gen6 only uses geometry shaders for transform feedback so the state
we emit is not suitable to accomodate general purpose, user-provided geometry
shaders. This patch paves the way to add these support and the needed
3DSTATE_GS packet modifications for it.

Previous code that emitted state to implement transform feedback in gen6 goes
to upload_gs_state_adhoc_tf().

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