drm/omap: fix: Defer probe if an omapdss device requests for it at connect

drm/omap: fix: Defer probe if an omapdss device requests for it at connect

With the omapdss device model changes. omapdrm is required to call dssdriver's
connect() op to register a panel. This is currently done in omap_modeset_init()

A call to connect() can fail if the omapdss panels or the encoders(HDMI/DPI)
they connect to have some resource(like regulators, I2C adapter) missing. If
this happens, the correct approach is to defer omapdrm's probe.

omapdrm currently ignores those panels which return a non zero value when
connected. This could result in omapdrm ignoring all panels on an omap board.

The right approach would be for omapdrm to request for probe deferral when a
panel's connect op returns -EPROBE_DEFER.

In order to do this, we need to call connect() much earlier during omapdrm's
probe to prevent too many things are already done by then. We now connect the
panels during pdev_probe(), before anything else is initialized, so that we
don't need to undo too many things if a defer was requested.

Now when we enter omap_modeset_init(), we have a set of panels that have been
connected. We now proceed with registering only those panels that are already
connected.

A special case has to be considered when no panels are available to connect when
omapdrm probes. In this case too, we defer probe and expect that a panel will be
available to connect the next time.

Checking whether the panel has a driver or whether it has get_timing/read_edid
ops in omap_modeset_init() are redundant with the new display model. These can
be removed since a dssdev device will always have a driver associated with it,
and all dssdev drivers have a get_timings op.

This will mainly fix cases when omapdrm is built-in the kernel, since that's
generally where resources like regulators or I2C are unavailable because of
probe order dependencies.

In particular this fixes boot with omapdrm built-in on an omap4 panda ES board.
The regulators used by HDMI(provided by I2C based TWL regulators) aren't
initialized because I2C isn't initialized, I2C isn't initialized as it's pins
are not configured because pinctrl is yet to probe.

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