hevc_mvs: avoid deriving tmvp in amvp

Reduces on a sequence number of calls from 933685 to 586271.
Reviewed-by: Mickaël Raulet <mraulet@insa-rennes.fr>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/hevc_mvs.c

@@ -751,7 +751,8 @@ scalef:
         mvpcand_list[numMVPCandLX++] = mxB;
 
     //temporal motion vector prediction candidate
-    if (numMVPCandLX < 2 && s->sh.slice_temporal_mvp_enabled_flag) {
+    if (numMVPCandLX < 2 && s->sh.slice_temporal_mvp_enabled_flag &&
+        mvp_lx_flag == numMVPCandLX) {
         Mv mv_col;
         int available_col = temporal_luma_motion_vector(s, x0, y0, nPbW,
                                                         nPbH, ref_idx,