drm/i915: Refactor code to select the DDI buf translation table

drm/i915: Refactor code to select the DDI buf translation table

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: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170223173507.17600-1-ville.syrjala@linux.intel.com
Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>