drm/i915/guc: Assert ctch->vma is allocated

drm/i915/guc: Assert ctch->vma is allocated

Silence smatch by demonstrating that ctch->vma is allocated
following a successful chch_init()

drivers/gpu/drm/i915/intel_guc_ct.c:204 ctch_open() error:
 we previously assumed 'ctch->vma' could be null (see line 197)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171106135154.52520-1-michal.wajdeczko@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>