drm/xe/vm: fix double list add

It looks like the driver only wants to track one vma for each external
object per vm. However it looks like bo_has_vm_references_locked() will
ignore any vma that is marked as vma->destroyed (not actually destroyed
yet). If we then mark our externally tracked vma as destroyed and then
create a new vma for the same object and vm, we can have two externally
tracked vma for the same object and vm. When the destroy actually
happens it tries to move the external tracking to a different vma, but
in this case it is already being tracked, leading to double list add
errors. It should be safe to simply drop the destroyed check in
bo_has_vm_references(), since the actual destroy will switch the
external tracking to the next available vma.

Closes: #290


Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>