drm/i915: Drop bogus comment about RCPB unit clock gating on IVB

Someone copy pasted the comment from the SNB code w/o reading it.
We never actually implemented the workaround to disable RCPB unit
clock gating on IVB. It would have been needed for early steppings,
but we don't care about those anymore, so just remove the stale
comment.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>