drm/i915/dsi: Do not read the transcoder register.

As per the Bspec, port mapping is fixed for mipi dsi.

v2: Reuse the existing function (Jani)

Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191119072004.4093-1-vandita.kulkarni@intel.com