Use NUM_REF_FRAMES in update_ref_buffers( )

[ycho]

In the section E.5.1 Decoder model function of AV1 Spec,

update_ref_buffers ( idx, refresh_frame_flags ) {
for ( i = 0; i < 8; i++ ) {
==> I think we want to use defined symbol name NUM_REF_FRAMES instead of 8
for ( i = 0; i < NUM_REF_FRAMES; i++ ) {