mibstore.h and its miInitializeBackingStore() have been otherwise empty
since at least xorg-server 1.10. remove all reference to them as the
header is gone in xorg-server 1.18.

Make it work with xorg-server-1.10.6.

Initial import of xf86-video-intel 2.9.1.
This is the final version of the driver with UMS support.