These registers exist only before GEN5, so currently we may access
undefined registers on VLV/CHV and BXT. Apply the workaround only pre
GEN5.

Since the workaround is relevant only when LVDS is present, for clarity
apply it only if this is the case.

This triggered an unclaimed register access warning on BXT.

v2: (Ville)
- move the workaround to the LVDS init code
- print a debug note about the workaround
Signed-off-by: NImre Deak <imre.deak@intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>