drm/i915/bxt: limit WaDisableMaskBasedCammingInRCC to stepping A

Also make the WA comment consistent with the rest, where the stepping
info is not shown.

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