xfree86: Merge vbe into int10

There's not really a good reason to keep these separate, the vbe code
requires int10 and is not very large. This change eliminates the
build-time options for vbe; if you build int10, you get vbe.

Gitlab: xorg/xserver#692
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
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