i965/draw: Stop relying on min_index == -1 for invalid index bounds

i965/draw: Stop relying on min_index == -1 for invalid index bounds

The vbo layer passes an index_bounds_valid flag that we should be using
instead.  This also fixes a bug when min_index == -1 and basevertex != 0
where we were actually comparing min_index + basevertex == -1 which was
false and we were getting the wrong buffer-sizing path.

Cc: "11.1 11.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>