gxfb: replace FBSIZE config option with a module parameter

Use a command line option (vram) rather than hardcoding the vram size.  LxFB
already does this; it's useful for machines that can't query the BIOS for fb
size.  This patch originated from David Woodhouse, was modified by Jordan
Crouse, and was then modified further by me.

This also adds some gxfb documentation in Documentation/fb.

Signed-off-by: Andres Salomon <dilinger@debian.org>
Cc: Jordan Crouse <jordan.crouse@amd.com>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>