fbdev: make offb driver tristate

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 <[e-mail]>
Suggested-by: Arnd Bergmann <[e-mail]>
Cc: Masahiro Yamada <[e-mail]>
Cc: Thomas Zimmermann <[e-mail]>
Cc: Michal Suchánek <[e-mail]>
Cc: [e-mail]
Cc: Daniel Vetter <[e-mail]>
Cc: Helge Deller <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Acked-by: Thomas Zimmermann <[e-mail]>
Signed-off-by: Helge Deller <[e-mail]>

drivers/video/fbdev/Kconfig

···
 	  chipset found in Ataris.
 
 config FB_OF
-	bool "Open Firmware frame buffer device support"
-	depends on (FB = y) && PPC && (!PPC_PSERIES || PCI)
+	tristate "Open Firmware frame buffer device support"
+	depends on FB && PPC && (!PPC_PSERIES || PCI)
 	depends on !DRM_OFDRM
 	select APERTURE_HELPERS
 	select FB_CFB_FILLRECT