drm/xe: do not register to PM if GuC is disabled

When working without GuC (i.e. working with execlists), the flow
attempts to perform suspend operation which is failing due to a
lack of support without GuC.

If PM ops are not supported without GuC we may as well avoid PM
registration rather than returning errors from various PM flows.

Signed-off-by: Ohad Sharabi <osharabi@habana.ai>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>