drm/udl: Remove field lost_pixels from struct udl_device

The field lost_pixels in struct udl_device was supposed to signal an
error during USB transfers of the framebuffer data. The driver would
have to schedule a re-transfer at a later point. This code was never
implemented. Remove lost_pixels and return regular error codes instead.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191206085954.9697-8-tzimmermann@suse.de