drm/omap: squash omapdrm sub-modules into one

At the moment we have three different modules: omapdss-base, omapdss,
omapdrm. This setup is finally obsolete, as the last omapdrm specific
panel has been converted to DRM panel.

We can thus remove omapdss-base and omapdss, and just compile everything
into omapdrm.ko.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-51-tomi.valkeinen@ti.com