drm/xe/migrate: Switch from drm to dev managed actions

drm/xe/migrate: Switch from drm to dev managed actions

Change the scope of the migrate subsystem to be dev managed instead of
drm managed.

The parent pci struct &device, that the xe struct &drm_device is a part
of, gets removed when a hot unplug is triggered, which causes the
underlying iommu group to get destroyed as well.

The migrate subsystem, which handles the lifetime of the page-table tree
(pt) BO, doesn't get a chance to keep the BO back during the hot unplug,
as all the references to DRM haven't been put back.
When all the references to DRM are indeed put back later, the migrate
subsystem tries to put back the pt BO. Since the underlying iommu group
has been already destroyed, a kernel NULL ptr dereference takes place
while attempting to keep back the pt BO.

Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/3914
Suggested-by: Thomas Hellstrom <thomas.hellstrom@intel.com>
Reviewed-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Signed-off-by: Aradhya Bhatia <aradhya.bhatia@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250326151929.1495972-1-aradhya.bhatia@intel.com
Signed-off-by: Tejas Upadhyay <tejas.upadhyay@intel.com>