drm/i915: Warn if trying to register eDP on port != B/C on vlv/chv

Only ports B and C have the power sequencer and backlight controls,
so complain if we ever try to register an eDP connector on some other
port.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>