drm/i915/guc: Assert guc->stage_desc_pool is allocated

drm/i915/guc: Assert guc->stage_desc_pool is allocated

Silence smatch by demonstrating that guc->stage_desc_pool is allocated
following a successful guc_stage_desc_pool_create(),

drivers/gpu/drm/i915/i915_guc_submission.c:1293 i915_guc_submission_init() error: we previously assumed 'guc->stage_desc_pool' could be null (see line 1261)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Oscar Mateo <oscar.mateo@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171106114833.31199-1-chris@chris-wilson.co.uk
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>