drm/i915: inline intel_best_encoder

Also kill the error-path, we have a fixed connector->encoder mapping.

Unfortunately we can't rip out all the ->best_encoder callbacks, these
are all still used by the fb_helper. Neat helper layering violation there.

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