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>
(cherry picked from commit 345f2234d1717d6128b2f90a7839c4906bf203ec)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>