Update DRM generic code to Linux 3.8

Currently, our DRM code in sys/dev/drm2 is a copy of the old DRI1 code from sys/dev/drm, plus the required bits from DRI2 to make the i915 KMS driver work. The missing parts were taken from Linux older than 3.4 (I don't know the exact version).

We need to update this DRM code to fix bugs and add new features, especially DRM-PRIME to support multi-cards configurations. This will allow us to ease current driver support (especially update the i915 driver to gain support for Haswell platforms) and port new drivers.

Here's a new version of the patch, after the merge of the i915 kernel driver update (committed in r277487 by Konstantin Belousov): http://people.freebsd.org/~dumbbell/graphics/drm-update-38.d.patch

It is still working fine with Radeon and appears to finally work with i915 too!

1. Clone the kms-drm-update-38 branch on GitHub: https://github.com/dumbbell/freebsd/tree/kms-drm-update-38
2. Build and install kernel
3. Reboot