staging/xgifb: Use structs and defines from the sis headers

This patch removes the usage of some xgi structs and defines and
replaces them with the _identical_ structs from the sis headers.
Thus the old structs and defines can be removed.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>