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

drivers/gpu/drm/omapdrm/Kconfig

@@ -2,6 +2,7 @@ config DRM_OMAP
 	tristate "OMAP DRM"
 	depends on DRM
 	depends on ARCH_OMAP2PLUS || ARCH_MULTIPLATFORM
+	select OMAP2_DSS
 	select DRM_KMS_HELPER
 	select DRM_KMS_FB_HELPER
 	select FB_SYS_FILLRECT