draw: fix primitive restart bug by using the index buffer offset

draw: fix primitive restart bug by using the index buffer offset

The code which scans the index buffer for restart indexes wasn't adding
the index buffer offset so we were always starting at offset=0.  The
offset is usually zero so it wasn't noticed before.

Fixes a failure in the piglit primitive-restart test when testing
vertex data + index data in a single VBO.

NOTE: This is a candidate for the 8.0 branch.