According to cLeAv0 on xda-developers, we should be able to use the nouveau driver with the NVEA (GK20A) GPU in the Nexus 9, if we backport the changes from 3.17 into the android-tegra-flounder-3.10-lollipop-release, or alternatively port Nexus 9 support into the mainstream 3.17 (or newer) kernel...
https://archive.fosdem.org/2015/schedule/event/tegra/
https://archive.fosdem.org/2015/schedule/event/tegra/