is_pch_edp() will be removed in a follow-up patch, so replace it
with a check for the port and VBT info (for port-D eDP).

Also make things a bit clearer by using a switch on the ports.

v2:
- make the comment about not setting the conder type for DP clearer
  (Ville)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>