drm/i915/guc: Unify parameters of public CT functions

There is no need to mix parameter types in public CT functions
as we can always accept intel_guc_ct.

v2: fix 'Return' doc, s/dev_priv/i915 (Sagar)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180320162020.38672-1-michal.wajdeczko@intel.com