ps3fb: Shrink default virtual frame buffer size from 18 to 9 MiB

ps3fb: Shrink the default virtual frame buffer size from 18 to 9 MiB, as
nobody really uses the double buffering feature and Linux can use an
additional 9 MiB.  It can still be overridden on the kernel command line using
`ps3fb=18M'.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Cc: Geoff Levand <geoffrey.levand@am.sony.com>
Cc: Paul Mackerras <paulus@samba.org>
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>