Use a locked xchg to ensure that the global log message giving
instructions on how to send a bug report is emitted precisely once.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NMika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190819075835.20065-5-chris@chris-wilson.co.uk