i965/gs: set up GS state
!UPSTREAM: turn on GS, and mark GS stuff as untested for safety
!UPSTREAM: allow GS through
Fixup GS_OPCODE_SET_VERTEX_COUNT
Fixup: Increment vertex count after emitting vertex
Fixup: output_vertex_size_owords
Fixup: use mlen=1 for GS thread end
Fixup gen7_upload_gs_push_constants
Fixup URB computation to avoid segfault