drm/i915/guc: Use GUC prefix for CORE_FAMILY definitions

Almost all other GuC fw definitions are using GUC|guc prefix.
While around, in get_core_family() change explicit WARN into MISSING_CASE
as it looks more appropriate, since GuC support capability we are controlling
by intel_device_info.has_guc flag.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170404133836.125736-1-michal.wajdeczko@intel.com
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>