drm/i915/guc: Don't clear the cookie on doorbell destroy

If the HW has not processed the db invalidation request yet, clearing
the cookie can generate a db ring. We clear the cookie when we
(re-)allocate the doorbell so no need to do it on destroy as well as no
one is going to look at it while the doorbell is inactive

v2: fix typo in patch title (Michal)

Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181002215430.15049-2-daniele.ceraolospurio@intel.com