drm/i915/selftests: Pretty print the i915_active

If the idle_pulse fails to flush the i915_active, dump the tree to see
if that has any clues.

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/20191031101116.19894-1-chris@chris-wilson.co.uk