drm/xe/device: Remove unused xe_device::usm::num_vm_in_*

Those counters were used to keep track of the numbers VMs in fault mode
and in non-fault mode, to determine if the whole device was in fault mode
or not. This is no longer needed so remove those variables and their
usages.

Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240809155156.1955925-12-francois.dugast@intel.com