drm/i915/ddi: Rely on VBT DDI port info for eDP detection

There is at least one APL based system using port A in DP mode
(connecting to an on-board DP->VGA adaptor). Atm we'll configure port A
unconditionally as eDP which is incorrect in this case. Fix this by
relying on the VBT DDI port 'internal port' flag instead on all ports on
DDI platforms. For now chicken out from using VBT for port A before
GEN9.

v2:
- Move the DDI port info lookup to intel_bios_is_port_edp() (David, Jani)
- Use the DDI port info on all DDI platforms starting from port B.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: David Weinehall <david.weinehall@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v1)
Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1482315444-24750-1-git-send-email-imre.deak@intel.com