drm/panel: panel-simple: get rid of panel_dpi hack

The empty panel_dpi struct was only ever used as a discriminant, but
it's kind of a hack, and with the reworks done in the previous patches,
we shouldn't need it anymore.

Let's get rid of it.

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-v2-5-5afcaa608bdc@kernel.org


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