xf86-video-xgi xf86-video-xgixp: Tinderbox build failures for big endian

[Jeremy Huddleston]

xf86-video-xgi and xf86-video-xgixp fail to build on BigEndian architectures due to:

http://tinderbox.x.org/builds/2011-03-25-0003/logs/xf86-video-xgi/#build
src/xgi_accel.c references undefined CurrentColorDepth

http://tinderbox.x.org/builds/2011-03-25-0003/logs/xf86-video-xgixp/#build
src/xgi_driver.c references undefined DRAWABLE_BUFFER

--Jeremy