drm/i915/bxt: support for HPD long/short status decoding

All non-GMCH platforms have the same register layout for HPD long/short
status, so let's use this condition instead of HAS_PCH_SPLIT, as the
latter doesn't apply for BXT.

Noticed by Daniel.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>