ARM: OMAP2+: fix missing variable declaration

The function that was added doesn't actually build:

arch/arm/mach-omap2/display.c: In function 'omapdss_init_fbdev':
arch/arm/mach-omap2/display.c:184:2: error: 'r' undeclared (first use in this function)

This adds a declaration for 'r' to fix it.

Fixes: 5ce78302

 ("ARM: OMAP2+: Don't register omapdss device for omapdrm")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>