lxfb: rename kernel arg fbsize to vram

Match other fb drivers (including gxfb).  Also, document the current boot
arguments in Documentation/fb/lxfb.txt.

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