vp9: remove one optimization branch in iadst16 which causes overflows.

See sample vp90-2-14-resize-fp-tiles-16-8-4-2-1.webm from the vp9 test
vector set which reproduces the issue. This probably costs a few cycles,
but I don't think there's an easy way to workaround that.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/x86/vp9itxfm.asm

@@ -1699,7 +1699,9 @@ VP9_IDCT_IDCT_16x16_ADD_XMM avx
     SUMSUB_BA                w,  5,  7,  4
     PSIGNW                  m5, [pw_m1]                     ; m12=out15[w], m8=t3[w]
 
-%if cpuflag(ssse3)
+    ; unfortunately, the code below overflows in some cases, e.g.
+    ; http://downloads.webmproject.org/test_data/libvpx/vp90-2-14-resize-fp-tiles-16-8-4-2-1.webm
+%if 0 ; cpuflag(ssse3)
     SUMSUB_BA               w,   7,  6,  4
     pmulhrsw                m7, [pw_m11585x2]               ; m8=out7[w]
     pmulhrsw                m6, [pw_11585x2]                ; m1=out8[w]