drm/amdgpu: move VM table mapping into the backend as well

Clean that up further and also fix another case where the BO
wasn't kmapped for CPU based updates.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>