[AGPGART] help text updates

This patch contains help text updates including the following:
- XFree86 * -> X
- there is no need for repeating part of the help text of the AGP
  option and having "If unsure, say Y/N." in the chip specific
  options.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Dave Jones <davej@redhat.com>