mn10300: use generic fb.h

The mn10300 arch uses a verbatim copy of the asm-generic version and
does not add any own implementations to the header, so use
asm-generic/fb.h instead of duplicating code.

Link: http://lkml.kernel.org/r/20170517083348.1815-1-tklauser@distanz.ch


Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>