drm/xe: Pre-allocate system memory for pinned external bos in the pm notfier

drm/xe: Pre-allocate system memory for pinned external bos in the pm notfier

Similarly to what we do for other pinned bos, pre-allocate
system memory for pinned external bos in the pm notifier,
where swapping is still possible.

This hasn't been needed until now when we're about to allow
pinning of exernal VRAM bos.

Cc: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://lore.kernel.org/r/20250918092207.54472-3-thomas.hellstrom@linux.intel.com