panfrost: Avoid copying job descriptors around when we can

panfrost: Avoid copying job descriptors around when we can

Job descriptors are written section by section and are never modified
after them been emitted. Let's avoid copying things around by allocating
descriptors upfront and letting the scoreboard logic only write the
header section.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6797>