drm/xe/vm: tidy up xe_runtime_pm usage

The xe_device_mem_access_get() should be all that's needed here and
should now work as expected, without any strange races. In theory should
be no functional changes here.

Reported-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>