drm/i915: Only emit the 'send bug report' once for a GPU hang

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: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190819075835.20065-5-chris@chris-wilson.co.uk