drm/i915/selftests: Show the RC6 residency on parking failure

Include the current RC6 residency counter in the error message, so that
if we fail to park and manually enter RC6 we can see if the counter has
a particularly suspect value (such as 0).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200123145755.1420622-1-chris@chris-wilson.co.uk