drm/i915/bxt: Correct dual-link MIPI port control.

For BXT, there is only one bit that enables/disables dual-link mode
and not different bits depending on which pipe is being used.

Signed-off-by: Bob Paauwe <bob.j.paauwe@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479767046-3964-1-git-send-email-bob.j.paauwe@intel.com


Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>