Merge omapdss compat layer work

We have two separate, exclusive, users of omapdss: 1) omapfb + omap_vout and 2)
omapdrm. Because omapfb and omap_vout are independent drivers, we've built
layers in omapdss to manage the two simultaneous callers. These layers are not
needed for omapdrm, as omapdrm is the sole user of omapdss, and these layers in
fact only create trouble for omapdrm.

The simple option to improve omapdrm situation would be to copy the omapdss
code for omapdrm. We are trying to avoid this, as omapdss and the panel drivers
are quite a lot of code together, and most of the code would be used without
change.

Thus this series helps the situation by moving the omapdss code required by
omapfb + omap_vout to separate files, creating a distinct layer used only by
omapfb + omap_vout. We call this layer "compat layer". This compat layer then
uses the core omapdss driver to operate the hardware. omapdrm will use the core
omapdss directly, without any layers in between.

After this series, omapfb, omap_vout and omapdrm can all be compiled at the
same time. Obviously omapdrm and omapfb+omap_vout cannot be run at the same
time (the first one to start will "win"), so compiling them at the same time is
only sensible as modules for testing purposes. Normal users should only compile
one of those.

This series does not make omapdrm use the core omapdss API, that will happen in
a separate series for omapdrm.