drm/omap: make omapdrm select OMAP2_DSS

Now that omapdss is only for omapdrm, we can change omapdrm to select
OMAP2_DSS to enable omapdss if omapdrm is enabled, instead of omapdrm
depending on omapdss.

We can also change omapdss and the display drivers to depend on
DRM_OMAP, so that they are only visible under omapdrm in menuconfig.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Acked-by: Rob Clark <robdclark@gmail.com>