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>

drivers/video/fbdev/mmp/Makefile

-obj-y += core.o hw/ panel/ fb/
+obj-$(CONFIG_MMP_DISP) += mmp_disp.o hw/ panel/ fb/
+
+mmp_disp-y		+= core.o

drivers/video/fbdev/mmp/fb/Kconfig

 if MMP_DISP
 
 config MMP_FB
-	bool "fb driver for Marvell MMP Display Subsystem"
+	tristate "fb driver for Marvell MMP Display Subsystem"
 	depends on FB
 	select FB_CFB_FILLRECT
 	select FB_CFB_COPYAREA