radeonfb: Add quirk for the graphics adapter in some JSxx

These devices are set to 640x480 by firmware, switch them to 800x600@60
so that the graphical installer can run on remote console.

Reported by IBM during SLES10 SP2 beta testing:

https://bugzilla.novell.com/show_bug.cgi?id=461002
LTC50817

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>