drm/i915/hdmi, dp: Do not dereference the encoder in the connector destroy

Oops, apparently intel_hdmi/intel_dp is the encoder - an object with a
distinct lifetime to the connector, and so we cannot simply reuse the
common function to unset and free the edid.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>