[AD] Asm bank switchers cleanup

["Eric Botcazou" <ebotcazou@xxxxxxxxxx>]

While scanning the asm bank switchers to detect conflicts with the i386
blitters patch, I took the opportunity to simplify and clean up the codes,
as well as to make them more consistent. The modified gfx drivers are:
- BeOS fullscreen and windowed,
- Linux SVGAlib,
- QNX fullscreen and windowed,
- Windows fullscreen, windowed and GDI,
- Unix X11 and XDGA2.

Tested on every driver except BeOS. Commited to trunk.

--
Eric Botcazou
ebotcazou@xxxxxxxxxx

Attachment: asm_switch.zip
Description: Zip compressed data