drm/i915: Allow DSI dual link to be configured on any pipe

Just like single link MIPI panels, similarly for dual link panels, pipe
to be configured is based on the DVO port from VBT Block 2. In hardware,
Port A is mapped with Pipe A and Port C is mapped with Pipe B.

This issue got introduced in -

commit 7e9804fd
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Fri Jan 16 14:27:23 2015 +0200

    drm/i915/dsi: add drm mipi dsi host support

Cc: stable@vger.kernel.org # v4.0
Signed-off-by: Gaurav K Singh <gaurav.k.singh@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>