drm/udl: Test pixel limit in mode-config's mode-valid function

The sku_pixel_limit is a per-device property, similar to the amount
of available video memory. Move the respective mode-valid test from
the connector to the mode-config structure.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221006095355.23579-3-tzimmermann@suse.de