x86/scale: fix xmm register count for hscale*_sse2

xmm6 was being clobbered in ff_hscale8to{15,19}_8_sse2 on Win64
Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswscale/x86/scale.asm

@@ -424,7 +424,7 @@ INIT_MMX mmx
 SCALE_FUNCS2 0, 0, 0
 %endif
 INIT_XMM sse2
-SCALE_FUNCS2 6, 7, 8
+SCALE_FUNCS2 7, 6, 8
 INIT_XMM ssse3
 SCALE_FUNCS2 6, 6, 8
 INIT_XMM sse4