draw: fix piglit base vertex + user vertex array tests

draw: fix piglit base vertex + user vertex array tests

This fixes
draw-elements-base-vertex user_varrays
draw-elements-instanced-base-vertex user_varrays
for softpipe with no llvm support (DRAW_USE_LLVM=false)

I'm not sure if this is the correct answer, but these tests were showing
a max_index of 7, then trying to fetch up to 43, maybe it should be fixing
max_index earlier somewhere to take care of this.

Signed-off-by: Dave Airlie <airlied@redhat.com>