drm/panel: panel-simple: Add function to look panel data up

Commit de04bb00 ("drm/panel/panel-simple: Use the new allocation in
place of devm_kzalloc()") moved the call to drm_panel_init into the
devm_drm_panel_alloc(), which needs a connector type to initialize
properly.

In the panel-dpi compatible case, the passed panel_desc structure is an
empty one used as a discriminant, and the connector type it contains
isn't actually initialized.

It is initialized through a call to panel_dpi_probe() later in the
function, which used to be before the call to drm_panel_init() that got
merged into devm_drm_panel_alloc().

So, we do need a proper panel_desc pointer before the call to
devm_drm_panel_alloc() now. All cases associate their panel_desc with
the panel compatible and use of_device_get_match_data, except for the
panel-dpi compatible.

In that case, we're expected to call panel_dpi_probe, which will
allocate and initialize the panel_desc for us.

Let's create such a helper function that would be called first in the
driver and will lookup the desc by compatible, or allocate one if
relevant.

Reported-by: Francesco Dolcini <francesco@dolcini.it>
Closes: https://lore.kernel.org/all/20250612081834.GA248237@francesco-nb/


Fixes: de04bb00 ("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-v2-4-5afcaa608bdc@kernel.org


Signed-off-by: Maxime Ripard <mripard@kernel.org>