drm/omap: move omapdss & displays under omapdrm

Now that omapfb has its own copy of omapdss and display drivers, we can
move omapdss and display drivers which omapdrm uses to omapdrm's
directory.

We also need to change the main drm Makefile so that omapdrm directory
is always entered, because omapdss has a file that can't be built as a
module.

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