https://bugzilla.kernel.org/show_bug.cgi?id=75401
--- Comment #5 from drill87@gmail.com --- I guess it fixes. At least, I can see no errors for radeon module in dmesg. Vgaswitcheroo, as I aforementioned, works too (I can turn off the card, so notebook heats less).
However, I cannot say any more, because I fail to actually use my radeon for rendering (with Xorg server 1.15.1) --- even command "glxinfo" with "DRI_PRIME=1" (and configured offload sink) crashes Xserver. And I don't know where the problem might be - in module or in Xorg drivers.