i965: Rework vertex uploads for Broadwell.

i965: Rework vertex uploads for Broadwell.

v2: Emit a dummy 3DSTATE_VF_SGVS packet when not needed.

v3: Add WARN_ONCE and perf_debugs requested by Eric Anholt.

v4: Program 3DSTATE_SGVS even in the no-elements case so gl_VertexID
    continues working.  Fix 3DSTATE_VF_INSTANCING to not use an
    element index to access the buffers array.  Some ARB_draw_indirect
    prep work.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>