drm/xe/mmio: grab mem_access in xe_mmio_ioctl

Any kind of device memory access should first ensure the device is not
suspended, mmio included.

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