arcfb: kill sparse warning

The framebuffer memory is allocated from system RAM (vmalloc'ed). Add __force
annotations.

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>