The loader_set_dispatch overwrites the magic with the dispatch
pointer, however when cmd buffers get recycled, and the loader
is in debug mode, it asserts that the magic isn't set anymore.

When recycling command buffers, reset the base object.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9145>