drm/simpledrm: Remove pdev field from device structure

Replace the remaining uses of the field pdev by upcasts from the Linux
device and remove the field.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220801135028.30647-4-tzimmermann@suse.de