Split the code to select the correct translation table into DP,
eDP and FDI specific helpers. This reduces the clutter in
intel_prepare_dp_ddi_buffers(), and we'll have other uses for some
of these new helper functions later on.

v2: Fix typo in commit message (David)
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170223173507.17600-1-ville.syrjala@linux.intel.comReviewed-by: NDavid Weinehall <david.weinehall@linux.intel.com>