drm/amdkfd: potential crash in kfd_create_indirect_link_prop()

This code has two bugs.  If kfd_topology_device_by_proximity_domain()
failed on the first iteration through the loop then "cpu_link" is
uninitialized and should not be dereferenced.

The second bug is that we cannot dereference a list iterator when it
points to the list head.  In other words, if we exit the
list_for_each_entry() loop exits without hitting a break then "cpu_link"
is not a valid pointer and should not be dereferenced.

Fix both of these problems by setting "cpu_link" to NULL when it is invalid
and non-NULL when it is valid.  That makes it easier to test for
valid vs invalid.

Fixes: 0f28cca8 ("drm/amdkfd: Extend KFD device topology to surface peer-to-peer links")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>