x86: Initialize mmxext after amd3dnow optimizations

The mmxext optimizations should be at least equally fast if available and
amd3dnow optimizations are being deprecated. Thus the former should
override the latter, not the other way around.
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  x86: Initialize mmxext after amd3dnow optimizations

Merged-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mjpegenc: use av_log2_16bit() in encode_block()

This reduces the number of operations done per non zero coefficient

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