vulkan/queue: Rework submit thread enabling

vulkan/queue: Rework submit thread enabling

Now that we have a threading mode in the device, we can set that based
on the environment variable instead of delaying it to submit time.  This
allows us to avoid the static variable trickery we use to avoid reading
environment variables over and over again.  We also move the enabling of
the submit thread up a level or two and give it a bit more obvious
condition.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15566>