gallium/pipebuffer: Use persistent maps for slabs

gallium/pipebuffer: Use persistent maps for slabs

Instead of the ugly practice of relying on the provider caching maps,
introduce and use persistent pipebuffer maps. Providers that can't handle
persistent maps can't use the slab manager.

The only current user is the svga drm winsys which always maps
persistently.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4804>