drm/i915: unload: fix intel dp encoder cleanup

struct intel_dp contains both struct intel_encoder at the beginning (as
it's base-class) and an i2c adapater. When initializing, the i2c adapter
gets assigned

	intel_encoder->ddc_adaptor = &intel_dp->adapter

and the generic intel_encode_destroy happily calls kfree on this pointer.
Ouch. Fix this by using a dp specific cleanup function.

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