d3d12: Move descriptor pools to screen, and add lock

d3d12: Move descriptor pools to screen, and add lock

Surfaces can be shared across contexts, and can even outlive the
original context that created them, so move the pools to the screen.
Since they no longer belong to a single context, they need a lock now.

v2: Samplers moved back to the context
v3: Fix Linux build

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3812
Reviewed-by: Bill Kristiansen <billkris@microsoft.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9349>