drm/i915: Assign request ringbuf before pin

In preparation to make intel_lr_context_pin|unpin to accept
requests, assign ringbuf into request before we call the pinning.

v2: No need to unset ringbuf on error path (Chris)

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