av1_nonrd_pick_inter_mode_sb: normalize ref frame check

Prefer `search_state.use_ref_frame_mask[]` over `cpi->ref_frame_flags`.
These are equivalent and checking the former is more consistent with the
rest of the function. This is a follow up to:
 4369bd1258 av1_nonrd_pick_inter_mode_sb: add missing ref_frame_flags check

Bug: 495477995, 495996858
Change-Id: Ie4bd1f4c80c4182add35c7a9c1977c15ce97d3bd

av1/encoder/nonrd_pickmode.c

diff --git a/av1/encoder/nonrd_pickmode.c b/av1/encoder/nonrd_pickmode.c
index 024dae6..14581d7 100644
--- a/av1/encoder/nonrd_pickmode.c
+++ b/av1/encoder/nonrd_pickmode.c
@@ -3448,7 +3448,7 @@
       !x->force_zeromv_skip_for_blk &&
       x->content_state_sb.source_sad_nonrd != kZeroSad &&
       x->source_variance == 0 && bsize < cm->seq_params->sb_size &&
-      (cpi->ref_frame_flags & AOM_LAST_FLAG) &&
+      search_state.use_ref_frame_mask[LAST_FRAME] &&
       search_state.yv12_mb[LAST_FRAME][0].width == cm->width &&
       search_state.yv12_mb[LAST_FRAME][0].height == cm->height) {
     set_block_source_sad(cpi, x, bsize, &search_state.yv12_mb[LAST_FRAME][0]);