indices: fix provoking vertex for quads/quadstrips

indices: fix provoking vertex for quads/quadstrips

This allows drivers to provide consistent flat shading for quads.
Otherwise a driver that only supported tris would have to force last
provoking vertex when drawing quads (and would have to say that quads
don't follow the provoking vertex convention).

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Rob Clark <robclark@freedesktop.org>