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

v2: Move call to drm_atomic_add_affected_connectors() to
    intel_modeset_compute_config(). (Daniel)
Signed-off-by: NAnder Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
[danvet: Resurrect the ret local variable which I've dropped from an
earlier patch and which is now needed.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>