It seems runpm is not working properly on your system. Booting with
radeon.runpm=0 reverts back to the 3.12 behavior (PX dGPUs are not dynamically
powered down). Did manually powering on/off the dGPU via debugfs ever work on
your system?  See the "Forcing the power state of the devices" section of this
page:
http://nouveau.freedesktop.org/wiki/Optimus/
for how to test that.