drm/tilcdc: Force building of DRM_TILCDC_SLAVE_COMPAT

If I read Documentation/kbuild/makefiles.txt section 3.6 right, this
patch should not be needed. However, without this patch the objects
needed for DRM_TILCDC_SLAVE_COMPAT are not linked, if DRM_TILCDC is
built as module.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>