xtensa: use generic fb.h

The 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/20170517083545.2115-1-tklauser@distanz.ch


Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>