PSR on skl is still disabled by default, because there's some corner cases we haven't fixed yet. In case you wonder: more recent platforms shut down more of the chip in PSR, and we haven't yet wired up the power management calls to wake it up again if we need it for those cases. Hopefully addressed in 4.7. Meanwhile you can enable it manually with i915.enable_psr=1 on the kernel cmdline and see whether it works in your case.