drm/i915/guc: Initialise doorbell cookie to matching value

Set the initial value of the doorbell cookie from the client.

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/20161129121024.22650-4-chris@chris-wilson.co.uk