drm/i915: access the PP_ON_DELAYS/PP_OFF_DELAYS regs only pre GEN5

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: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>