i965: Fix uploading user vertex arrays with basevertex set.

i965: Fix uploading user vertex arrays with basevertex set.

If the index buffer is full of values like "0 1 2 3", but basevertex is 4, we
need to upload at least vertex data for elements 4 5 6 7.  Whether we also
upload 0 1 2 3 is a question of whether there are VBOs present or not -- see
the code setting start_vertex_bias in brw_draw_upload.c.

Fixes piglit draw-elements*base-vertex user_varrays

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>