drm/i915/bxt: allow dsi on any pipe

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/0375f1e237092d0ae3f39ecfc5702024918acbfd.1458313400.git.jani.nikula@intel.com