vbo/dlist: fix loopback crash
The original code incorrectly adjusted only when Loopback was false, while primitives' start value is actually modified unconditionnally. Fixes: 32535942 ("vbo/dlist: rework buffer sizes") Closes: mesa/mesa#5754 Acked-by:Marek Olšák <marek.olsak@amd.com> Part-of: <mesa/mesa!14243> (cherry picked from commit 7a1d3d3a)