fb: convert /proc/fb to seq_file interface

Note: looks like accesses to "registered_fb" are done without any exclusion
so there're none in new proc code, too. This should be fixed in separate
patch.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.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>