vp9_diamond_search_sad_avx: quiet -Wmaybe-uninitialized warning

vp9_diamond_search_sad_avx: quiet -Wmaybe-uninitialized warning

w/gcc-11

v_these_mv_w is always initialized in this block with _mm_add_epi16();
converting this to a _mm_storeu_si32(tmp) call also works, but
introduces more stack usage

|| ../vp9/encoder/x86/vp9_diamond_search_sad_avx.c: In function
‘vp9_diamond_search_sad_avx’:
vp9/encoder/x86/vp9_diamond_search_sad_avx.c|285 col 19| warning:
‘v_these_mv_w’ may be used uninitialized [-Wmaybe-uninitialized]
||   285 |           new_bmv = ((const int_mv *)&v_these_mv_w)[local_best_idx];
||       |           ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
vp9/encoder/x86/vp9_diamond_search_sad_avx.c|149 col 21| note:
‘v_these_mv_w’ declared here
||   149 |       const __m128i v_these_mv_w = _mm_add_epi16(v_bmv_w, v_ss_mv_w);
||       |                     ^~~~~~~~~~~~

Change-Id: I1cd2fcb41030db16f51c94f3a70eb8eb2a526401