drm/i915: Use atomic state in intel_ddi_crtc_get_new_encoder()

Instead of using connector->new_encoder, get the same information from
the pipe_config, thus making the function ready for the atomic
conversion.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>