drm/i915: rip out superflous is_dp&is_cpu_edp tracking

The only exception left is is_cpu_edp in the haswell modeset code.
We need that to assign the cpu transcoder, but we might want to
move that eventually into the encoder, too.

\o/-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>