drm/i915: Allow eDP on port C in theory

The power sequencer has bits to allow DP C to be used for eDP.
Currently we assume this will never happen, but I guess it could
theoretically be a thing. Make the code do the right thing in that
case, and toss in a MISSING_CASE() for any other port.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180518152931.13104-5-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>