drm/i915: Don't use encoder->new_crtc in intel_lvds_compute_config()

Move towards atomic by using the legacy modeset's drm_atomic_state
instead.

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