"Modesetting" requires special versions of software to work. If you enable the kernel modesetting support for your hardware, but you don't have a modesetting-capable X.org driver, YOUR MACHINE WILL NOT WORK (it even can crash). So, if you don't have such drivers, it's not worth even thinking about enabling the kernel modesetting support.
Right now, there're not many stable releases of drivers with support for modesetting. So a X.org update will not be enought, you are required to download and compile the unstable development versions.
See http://dri.freedesktop.org/wiki/Building for building the drm and mesa bits.
- libdrm from git master of mesa/drm
- mesa from git master of mesa/mesa
- xf86-video-intel from git master
- drm-intel-next from anholt's kernel tree (git://git.kernel.org/pub/scm/linux/kernel/git/anholt/drm-intel)
You'll need to use UXA as your AccelMethod.
If you run into issues with the above, please file bugs (if you don't already see your issue filed) at bugs.freedesktop.org.