x86/vp9lpf: use fewer instructions in SPLATB_MIX

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/x86/vp9lpf.asm

@@ -302,9 +302,8 @@ SECTION .text
     pshufb     %1, %2
 %else
     punpcklbw  %1, %1
-    punpcklqdq %1, %1
-    pshuflw    %1, %1, 0
-    pshufhw    %1, %1, 0x55
+    punpcklwd  %1, %1
+    punpckldq  %1, %1
 %endif
 %endmacro