drm/i915/guc: init submission structures as part of guc_init

guc->stage_desc_pool is required as part of the init parameters and
there is no reason we have to init them after HuC. This fixes a NULL
ptr dereference due to guc->stage_desc_pool not being set (no fixes
tag since GuC submission can't be enabled yet).

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
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/20190725174655.24382-1-daniele.ceraolospurio@intel.com