drm/i915: Document that we are implementing WaDisableBackToBackFlipFix

For the next person that checks these kind of things, without having to
dig up the register definition.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>