In moving batch cache to the context, the check for whether there's
pending work being done to this resources ends up accessing the context,
so we can't do it outside of the fd_context_access_begin(). This flag
lets us do the driver-thread asserts before we've decided whether we need
to flush.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11511>
c3d6493122