Make the offb (Open Firmware frame buffer) driver tristate,
i.e., so that it can be built as a loadable module.

However, it still depends on the setting of DRM_OFDRM
so that both of these drivers cannot be builtin at the same time
nor can one be builtin and the other one a loadable module.

Build-tested successfully with all combination of DRM_OFDRM and FB_OF.

This fixes a build issue that Michal reported when FB_OF=y and
DRM_OFDRM=m:

powerpc64-linux-ld: drivers/video/fbdev/offb.o:(.data.rel.ro+0x58): undefined reference to `cfb_fillrect'
powerpc64-linux-ld: drivers/video/fbdev/offb.o:(.data.rel.ro+0x60): undefined reference to `cfb_copyarea'
powerpc64-linux-ld: drivers/video/fbdev/offb.o:(.data.rel.ro+0x68): undefined reference to `cfb_imageblit'
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Michal Suchánek <msuchanek@suse.de>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Helge Deller <deller@gmx.de>