Great thanks should be given to Taylor R Campbell for the work updating drm2 to Linux kernel 3.15. Perhaps too much has been done in that direction, but is it possible that Linux kernel 3.14 should be considered as the baseline since it is LTS, which to my understanding means updates for 2 years including updates to select drivers. Whereas if 3.15 is not LTS it could be rapidly superseded. If the plan is to make drm2 part of NetBSD 7, using 3.14 LTS is perhaps the easiest way to support the drm2 drivers at least a couple of the years NetBSD 7 will be supported.
Furthermore I believe both Debian unstable and Fedora 19 have 3.14 as their most recent kernels so that people debugging drm2 right now before the NetBSD 7 beta announcement could have systems actually running 3.14 even if it is a different OS.