Atomic modesetting: now with modesetting support.

v2: Moved drm_atomic_set_mode_prop_for_crtc from previous patch; removed
    state->active fiddling, documented return code. Changed property
    type to DRM_MODE_PROP_BLOB.
Signed-off-by: Daniel Stone <daniels@collabora.com>
Tested-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>