d3d12: Add a list of contexts alive for the current screen

d3d12: Add a list of contexts alive for the current screen

When a resource is destroyed, we'll need to let the contexts know.
This is guarded by the submit mutex, because we'll already be holding
that for at least one place where we want to iterate this list, and
it's low-frequency enough that re-using it is simpler than adding more
locks and creating confusing lock ordering.

Reviewed-by: Bill Kristiansen <billkris@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17688>