drm/i915/selftests: Try to show where the pulse went

We have a case of a mysteriously absent pulse, so dump the engine
details to see if we can find out what happened to it.

References: https://bugs.freedesktop.org/show_bug.cgi?id=112405


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/20191128102546.3857140-1-chris@chris-wilson.co.uk