Revert "drm/omap: no need to select OMAP2_DSS"

This reverts commit 1c278e5e.

If DRM_OMAP does not select OMAP2_DSS it is possible to build a kernel with
DRM_OMAP only and not selecting OMAP2_DSS. Since omapdrm depends on
OMAP2_DSS this will result on broken kernel build.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>