vbo: Only mark merged line strips as lines when actually converting them

vbo: Only mark merged line strips as lines when actually converting them

We only convert line strips to lines in certain cases, but were flagging
node->merged.prim as GL_LINES even if we simply copied a GL_LINE_STRIP
prim[0] over without modifying it.

Fixes Piglit's lineloop test (which triggers loop -> strip conversion
earlier in this path, then was incorrectly triggering strip -> list
mode modification with no changes to the underlying data).

Fixes: 310991415ee ("vbo/dlist: implement primitive merging")
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8522>