media: omap2: omapfb: allow building it with COMPILE_TEST

This driver builds cleanly with COMPILE_TEST, and it is
needed in order to allow building drivers/media omap2
driver.

So, change the logic there to allow building it.

Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>