rust/drm: Add UnbindGuard for drm_dev_enter/exit critical sections

DRM ioctls do not guarantee that the parent bus device is still bound.
However, since DRM device registration is managed through Devres, using
drm_dev_unplug() on unregistration ensures that between drm_dev_enter()
and drm_dev_exit() the parent device must be bound.

Add UnbindGuard, a guard object representing a drm_dev_enter/exit SRCU
critical section that dereferences to &Device<Bound> of the parent bus
device. The guard is only available on Device<T, Registered>, ensuring
it cannot be used on unregistered devices.

Also add with_unbind_guard() as a convenience helper that executes a
closure with the bound device reference.

Switch Registration::drop from drm_dev_unregister() to drm_dev_unplug()
to provide the SRCU barrier that UnbindGuard's safety argument relies on.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>