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.

printf format fixes, found using clang -Wformat.
Some are fixed by adding casts, some by changing the format string,
and some by changing the type of the variable.

make this compile on NetBSD, still needs to be tested on actual hardware
( hi msaitoh@ )

initial import of xf86-video-xgi-1.6.0