multimedia/libvpx: unbreak on amd64 after r454153

nasm -Ox -f elf64 -I./ -I"/wrkdirs/usr/ports/multimedia/libvpx/work/libvpx-1.6.1"/ -o vpx_dsp/x86/intrapred_sse2.asm.o vpx_dsp/x86/intrapred_sse2.asm
vpx_dsp/x86/intrapred_sse2.asm:64: error: invalid combination of opcode and operands
third_party/x86inc/x86inc.asm:1383: ... from macro `movd' defined here
third_party/x86inc/x86inc.asm:1283: ... from macro `RUN_AVX_INSTR' defined here

Reported by: antoine
Pointy hat to: jbeich