drm/amdkfd: Map wptr BO to GART unconditionally

For simulation C models that don't run CP FW where adev->mes.sched_version
is not populated correctly. This causes NULL dereference in
amdgpu_amdkfd_free_gtt_mem(dev->adev, (void **)&pqn->q->wptr_bo_gart)
and warning on unpinned BO in amdgpu_bo_gpu_offset(q->properties.wptr_bo).

Compared with adding version check here and there,
always map wptr BO to GART simplifies things.

v2: Add NULL check in amdgpu_amdkfd_free_gtt_mem.(Philip)

Signed-off-by: Lang Yu <lang.yu@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>