Blackfin: fix framebuffer mmap bug for nommu

The patch added a special get_unmapped_area for framebuffer which
was hooked to the file ops in drivers/video/fbmem.c.

This is needed since v2.6.29-rc1 where nommu vma management was
updated, and mmap of framebuffer caused kernel BUG panic. You may turn
on "Debug the global anon/private NOMMU mapping region tree" config to
such message.

As Documentation/nommu-mmap.txt said,
"To provide shareable character device support, a driver must provide
a file->f_op->get_unmapped_area() operation. The mmap() routines will
call this to get a proposed address for the mapping."

With this change, user space should call mmap for framebuffer using
shared map. Or it can try shared map first, then private map if
failed. This shared map usage is now consistent between mmu and nommu.

The sys_ file may not be a good place for this patch. But there is a
similar one for sparc. I tested a similar patch on nios2nommu, though
I don't have a blackfin board to test.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>