Fix MMX rgb24 to yuv conversion with gcc 4.6

When built with gcc 4.6, the MMX rgb24 to yuv conversion gives
wrong output.  The compiler produces this warning:

libswscale/swscale_template.c:1885:5: warning: use of memory input without lvalue in asm operand 4 is deprecated

Changing the memory operand to a register makes it work.

Signed-off-by: Mans Rullgard <mans@mansr.com>