drm: Use atomic state for FB in legacy ioctls

If atomic state is available, use this to read the current plane in
GetCrtc/GetPlane, rather than the legacy points.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161213181912.92904-1-daniels@collabora.com