drm/amdgpu: Correctly use bo_va->ref_count in compute VMs

This is needed to correctly handle BOs imported into compute VM from gfx.
Both kfd and gfx should use same bo_va and set bo_va->ref_count correctly
when map the Bos into same VM, otherwise we may trigger kernel general
protection when iterate mappings over bo_va's valids or invalids list.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Xiaogang Chen <Xiaogang.Chen@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ramesh Errabolu <Ramesh.Errabolu@amd.com>
Tested-by: Xiaogang Chen <Xiaogang.Chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>