gxfb/lxfb: detect framebuffer size using an MSR if VSA2 isn't available

If there's no VSA2 (ie, if we're using tinybios or OpenFirmware), use the
GLIU's P2D Range Offset Descriptor to determine how much memory we have
available for the framebuffer.

Originally based on a patch by Jordan Crouse.  Tested with OpenFirmware;
Pascal informs me that tinybios has a stub that fills in P2D_RO0.

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>