drm/panel: panel-simple: make panel_dpi_probe return a panel_desc

drm/panel: panel-simple: make panel_dpi_probe return a panel_desc

If the panel-simple driver is probed from a panel-dpi compatible, the
driver will use an empty panel_desc structure as a descriminant. It
will then allocate and fill another panel_desc as part of its probe.

However, that allocation needs to happen after the panel_simple
structure has been allocated, since panel_dpi_probe(), the function
doing the panel_desc allocation and initialization, takes a panel_simple
pointer as an argument.

This pointer is used to fill the panel_simple->desc pointer that is
still initialized with the empty panel_desc when panel_dpi_probe() is
called.

Since commit de04bb0089a9 ("drm/panel/panel-simple: Use the new
allocation in place of devm_kzalloc()"), we will need the panel
connector type found in panel_desc to allocate panel_simple. This
creates a circular dependency where we need panel_desc to create
panel_simple, and need panel_simple to create panel_desc.

Let's break that dependency by making panel_dpi_probe simply return the
panel_desc it initialized and move the panel_simple->desc assignment to
the caller.

This will not fix the breaking commit entirely, but will move us towards
the right direction.

Fixes: de04bb0089a9 ("drm/panel/panel-simple: Use the new allocation in place of devm_kzalloc()")
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Francesco Dolcini <francesco.dolcini@toradex.com> # Toradex Colibri iMX6
Link: https://lore.kernel.org/r/20250626-drm-panel-simple-fixes-v2-2-5afcaa608bdc@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>