video/mmpfb: allow modular build

The frame buffer core can be a module, which means any fb drivers
should be able to build as modules too. This turns mmpfb into
a tristate option to allow that and fix a possible randconfig
build error.

drivers/built-in.o: In function `modes_setup':
:(.text+0x11b34): undefined reference to `fb_videomode_to_modelist'
:(.text+0x11b5c): undefined reference to `fb_videomode_to_var'

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>