drm/amdgpu: separate bo_va structure

Split that into vm_bo_base and bo_va to allow other uses as well.

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