drm/xe/evict: drop bogus assert

drm/xe/evict: drop bogus assert

This assert can trigger here with non pin_map users that select
LATE_RESTORE, since the vmap is allowed to be NULL given that
save/restore can now use the blitter instead. The check here doesn't
seem to have much value anymore given that we no longer move pinned
memory, so any existing vmap is left well alone, and doesn't need to be
recreated upon restore, so just drop the assert here.

Fixes: 86f69c26113c ("drm/xe: use backup object for pinned save/restore")
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6213
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://lore.kernel.org/r/20251010152457.177884-2-matthew.auld@intel.com