drm/xe: Do not take any action if our device was removed.

Unfortunately devcoredump infrastructure does not provide and
interface for us to force the device removal upon the pci_remove
time of our device.

The devcoredump is linked at the device level, so when in use
it will prevent the module removal, but it doesn't prevent the
call of the pci_remove callback. This callback cannot fail
anyway and we end up clearing and freeing the entire pci device.

Hence, after we removed the pci device, we shouldn't allow any
read or free operations to avoid segmentation fault.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>