drm/amdkfd: fix null queue check on debug setting exceptions

Null check should be done on queue struct itself and not on the
process queue list node.

Signed-off-by: Jonathan Kim <jonathan.kim@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>