drm/i915: Only show debug for state changes when banning

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