drm/i915/bxt: fix dsi hw state pipe readout

BXT isn't as limited as BYT and CHT regarding DSI pipes and ports.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/8bea85b86aaf7a15f854a656bf8d3208a3afe0bd.1458070700.git.jani.nikula@intel.com