drm/i915/guc: Name the default GuC scheduling policy

drm/i915/guc: Name the default GuC scheduling policy

The default values for the default scheduling policy come from the
GuC firmware itself. Transform the magic numbers into defines.

Suggested-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1505252197-27696-1-git-send-email-oscar.mateo@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>