drm/atomic: Add dev pointer to drm_private_obj

drm/atomic: Add dev pointer to drm_private_obj

All the objects that need to implement some callbacks in KMS have a
pointer in there structure to the main drm_device.

However, it's not the case for drm_private_objs, which makes it harder
than it needs to be to implement some of its callbacks. Let's add that
pointer.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Tested-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Link: https://patch.msgid.link/20251014-drm-private-obj-reset-v2-1-6dd60e985e9d@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>