Rework drm2 configuration and modularization.

- Split drm2.kmod into drmkms_linux, drmkms, drmkms_pci.
- Split up drmkms-related files.* similarly.
- Make drm_agp_* routines hooks that the drmkms_pci module installs.
- Reuse drm device major 180 for (old) drm and drmkms.

Now old drm and new drmkms can coexist happily in the same tree, with
or without PCI, and require no changes to userland.
Remove drm2 and i915drm2 modules.
do not KASSERT out on Sandy Bridge
revert to bogus assertion, as requested by riastradh@ to ease forecoming commit