drm/i915: Drop unneed i915 parameter from intel_ring_pin()

As we now have a ring->vma available, we can just lookup our i915
pointer from inside the vm, and so not require the unsightly parameter.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180727155501.18963-1-chris@chris-wilson.co.uk