drm/i915/guc: Take enable_guc_loading check out of GEM core code

The should happen as soon as possible, but always within the logic that
depends on it (and not interrupting the top-level driver control flow).

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1490720027-23234-1-git-send-email-oscar.mateo@intel.com