drm/i915/guc: suppress GuC-related message on non-GuC platforms

If the user doesn't override the default values of the GuC-related
kernel parameters, then on a non-GuC-based platform we shouldn't
mention that we haven't loaded the GuC firmware.

The various messages have been reordered into a least->most severe
cascade (none/INFO/INFO/ERROR) for ease of comprehension.

Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465575685-34169-1-git-send-email-david.s.gordon@intel.com