drm/atomic: Fix remaining places where !funcs->best_encoder is valid

Adapt drm_pick_crtcs() and update_connector_routing() to fallback to
drm_atomic_helper_best_encoder() if funcs->best_encoder() is NULL so
that DRM drivers can leave this hook unassigned if they know they want
to use drm_atomic_helper_best_encoder().

Update the vtables documentation accordingly.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465300095-16971-2-git-send-email-boris.brezillon@free-electrons.com