drm/i915/guc: Introduce USES_GUC_xxx helper macros

In the upcoming patch we will change the way how to recognize
when GuC is in use. Using helper macros will minimize scope
of that changes. While here, update dev_info message.

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>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171206135316.32556-3-michal.wajdeczko@intel.com