drm/i915/guc: general tidying up (submission)

Renaming to more consistent scheme, and updating comments, mostly
about i915_guc_wq_reserve(), aka i915_guc_wq_check_space().

Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473711577-11454-4-git-send-email-david.s.gordon@intel.com


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>