Fix bug preventing X server from restarting.

The core DRM lastclose routine automatically destroys all mappings and
releases SG memory.  XP10 DRM and DDX assumed this data stayed around
until module unload.  xgi_bootstrap was reworked to recreate all these
mappings.  In addition, the drm_addmap for the GART backing store was
moved into the kernel.  This causes a change to the ioctl protocol and
a version bump.