drm/i915: Onion unwind for intel_init_ring_common()

Rather than call intel_engine_cleanup() with a partially constructed
engine, unwind the error during intel_init_ring_common().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170403113426.25707-2-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>