Allow a privacy screen provider to stash its private data pointer in the
drm_privacy_screen, and update the drm_privacy_screen_register() call to
accept that. Also introduce a *_get_drvdata() so that it can retrieved
back when needed.

This also touches the IBM Thinkpad platform driver, the only user of
privacy screen today, to pass NULL for now to the updated API.
Signed-off-by: Rajat Jain <rajatja@google.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220107190208.95479-1-rajatja@google.com