drm/i915: Fixup kerneldoc for struct intel_context

   commit ae6c4806
   Author: Daniel Vetter <daniel.vetter@ffwll.ch>
   Date:   Wed Aug 6 15:04:53 2014 +0200

       drm/i915: Only track real ppgtt for a context

Changed the code but didn't update kerneldoc.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: "Thierry, Michel" <michel.thierry@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>