drm/i915: kerneldoc for fences

v2: Clarify that this is about fence _registers_. Also clarify that
the fence code revokes cpu ptes and not gtt ptes. Both suggested by
Chris.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>