i965: Don't try to continue space before the start of a VBO.

i965: Don't try to continue space before the start of a VBO.

This loop is trying to see if all the buffers to be uploaded happen to
be the same increment from the start of the 3DSTATE_VERTEX_BUFFERS
currently loaded in the hardware.  However, we might be at a smaller
offset than the previous set of VERTEX_BUFFERS, so we can't reuse
because that packet made the first entry be its starting offset (you
can't access outside the given bounds).

Fixes piglit ARB_vertex_buffer_object/elements-negative-offset.