drm/i915/guc: Rename client->cookie to match use

The client->cookie is a shadow of the doorbell->cookie value, so rename
it to indicate its association with the doorbell, like the doorbell id
and offset.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161129121024.22650-3-chris@chris-wilson.co.uk