drm/i915: Add references to some workaround we implement

We did not mention the workaround name when implementing those. This
should help us track what we already implement.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>