drm/amdgpu: fix race in GEM VA map IOCTL v2

Somebody could try to free the bo_va between mapping and updating it.

v2: fix typos in comment

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>