OMAP: DSS2: OMAPFB: Allow FB_OMAP2 to build without VRFB

FB_OMAP2 can work without VRFB, but currently does not build. Fix this.

Signed-off-by: Senthilvadivu Guruswamy <svadivu@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>