drm/atomic: Handle funcs->best_encoder == NULL case

Fallback drm_atomic_helper_best_encoder() is 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().

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/20160601180337.28e0917b@bbrezillon