media: omapfb: omapfb_dss.h: add stubs to build with COMPILE_TEST && DRM_OMAP
Add stubs for omapfb_dss.h, in the case it is included by
some driver when CONFIG_FB_OMAP2 is not defined, with can
happen on ARM when DRM_OMAP is not 'n'.
That allows building such driver(s) with COMPILE_TEST.