drm/xe: Don't grab runtime PM ref in engine create IOCTL

A VM had a runtime PM ref, a engine can't be created without a VM, and
the engine holds a ref to the VM thus this is unnecessary. Beyond that
taking a ref in the engine create IOCTL and dropping it in the destroy
IOCTL is wrong as a user doesn't have to call the destroy IOCTL (e.g.
they can just kill the process or close the driver FD). If a user does
this PM refs are leaked.

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