drm/i915/selftests: Show the i915_active on failure

Print the i915_active state on selftest failure, with a hope it helps
illuminate the cause of the failure.

References: https://gitlab.freedesktop.org/drm/intel/issues/765


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210115502.3767070-1-chris@chris-wilson.co.uk