drm/i915: Assert that TypeC ports are not used for eDP

Add an assert that we don't use TypeC ports for eDP. That may in theory
be possible on TypeC legacy ports, but I'm not sure if that's a
practical scenario, so let's deal with that only if there's a use case.
Adding support for that wouldn't be too difficult, since TypeC mode
switching is not possible on TypeC legacy ports.

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190509173446.31095-12-imre.deak@intel.com