st/mesa: fix max_offset computation for base vertex

Add the maximum base vertex offset to max_index for computing the
buffer size.  Fixes a failed assertion in the u_upload_mgr.c code with
the VMware svga driver.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=48141

v2: incorporate Marek's suggestions.

NOTE: This is a candidate for the 8.0 branch.

Reviewed-by: Marek Olšák <maraeo@gmail.com>
1 file changed