drm/i915/guc: local optimisations and updating comments

Tidying up guc_init_proc_desc() and adding commentary to the client
structure after the recent change in GuC page mapping strategy.

Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461078516-28678-1-git-send-email-david.s.gordon@intel.com


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>