Got it. But what exactly premultiplying alpha does on pixmap? It does not seem to modify pixmap data at all. Since pmAlpha is using DESTINATION RGB to calculate resulting pixel color then it seems like premultiplying alpha is just some kind of flag/parameter which is used by device rendering part. And it seems to work globaly, not locally for one pixmap.
At least in all examples i saw the pmAlpha is set for d3d globally.