drm/simpledrm: Remove mem field from device structure

Remove the unused mem field from struct simpledrm_device.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220801135028.30647-2-tzimmermann@suse.de