dsputil: fix optimized emu_edge function on Win64.

Recent register allocation changes (x86inc.asm update) changed the
register order and thus opcodes for the inner loops. One of them became
>128bytes, which confuses other parts of this function where it jumps
to fixed-offset positions to extend the edge by fixed amounts. A simple
register change fixes this.

libavcodec/x86/dsputil_yasm.asm

@@ -594,7 +594,7 @@ cglobal emu_edge_core, 2, 7, 0
 %define valw2 r7w
 %define valw3 r3w
 %if WIN64
-%define valw4 r4w
+%define valw4 r7w
 %else ; unix64
 %define valw4 r3w
 %endif