drm/xe: Call exit functions when xe_register_pci_driver() fails

Move xe_register_pci_driver() and xe_unregister_pci_driver() to
init_funcs to make sure that exit functions are also called when
xe_register_pci_driver() fails.

Note that this also allows adding init functions to be run after
xe_register_pci_driver().

v2:
 - Move functions to init_funcs instead of having a special case for
   xe_register_pci_driver(). (Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Matt Atwood <matthew.s.atwood@intel.com>
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>