drm/i915/guc: Switch to CT_ERROR in ct_read

As we now have "ct" available in ct_read function we can switch
from generic DRM_ERROR to our custom CT_ERROR.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200117082039.65644-5-michal.wajdeczko@intel.com