radeonfb: fix 16bpp copyarea() bug

It turns out the bug was due to my mis-copying a certain set of flags from
the x.org tree.
Signed-off-by: David Eger <eger@havoc.gtf.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>