drm/i915/dp: use native encoder ->mode_set callback

Usual drill applies. Again I've not switched the upcast helpers to use
intel_encoder instead of drm_encoder since that's much more invasive
and will change also the hdmi and ddi encoders.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>