u_trace: Add an interface for checking trace enablement outside a context.

u_trace: Add an interface for checking trace enablement outside a context.

For zink, we want to know if we should pass command stream markers down to
the underlying driver, but we don't have our own trace context we're
recording trace events with.  We definitely want those markers if the
underlying driver is going to be doing perfetto tracing, or is requesting
marker tracing.  So, create an interface for querying those flags before
they get copied down to an actual u_trace_context.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20657>