Since we trigger 10,000s of hangs and resets during selftesting, we emit
many, many thousands of lines of useless debug messages. Reduce the
frequency by only logging a change in state of a guilty context.

Fixes: 14921f3c ("drm/i915: Fix context ban and hang accounting for client")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180618073135.10849-1-chris@chris-wilson.co.uk