drm/i915/execlists: Add interrupt-pending check to intel_execlists_idle()

Primarily this serves as a sanity check that the bit has been cleared
before we suspend (and hasn't reappeared after resume).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Tvrtko Ursulin <tursulin@ursulin.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170201131222.11882-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>