alpha: use generic fb.h

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

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Matt Turner <mattst88@gmail.com>