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.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>