drm/i915: Rename ring->outstanding_lazy_request

Prior to preallocating an request for lazy emission, rename the existing
field to make way (and differentiate the seqno from the request struct).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>