OMAPDSS: Implement display (dis)connect support

OMAPDSS: Implement display (dis)connect support

We currently have two steps in panel initialization and startup: probing
and enabling. After the panel has been probed, it's ready and can be
configured and later enabled.

This model is not enough with more complex display pipelines, where we
may have, for example, two panels, of which only one can be used at a
time, connected to the same video output.

To support that kind of scenarios, we need to add new step to the
initialization: connect.

This patch adds support for connecting and disconnecting panels. After
probe, but before connect, no panel ops should be called. When the
connect is called, a proper video pipeline is established, and the panel
is ready for use. If some part in the video pipeline is already
connected (by some other panel), the connect call fails.

One key difference with the old style setup is that connect() handles
also connecting to the overlay manager. This means that the omapfb (or
omapdrm) no longer needs to figure out which overlay manager to use, but
it can just call connect() on the panel, and the proper overlay manager
is connected by omapdss.

This also allows us to add back the support for dynamic switching
between two exclusive panels. However, the current panel device model is
not changed to support this, as the new device model is implemented in
the following patches and the old model will be removed. The new device
model supports dynamic switching.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>