Make the private array used for stashing test clients static, to silence
sparse.

References: 55bd6bd7 ("drm/i915/selftests: Add a GuC doorbells selftest")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171120132606.4254-1-chris@chris-wilson.co.ukReviewed-by: Michel Thierry <michel.thierry@intel.com>