drm/amdgpu: add support for BO_VAs without BO v2

For PRT support we need mappings which aren't backed by any memory.

v2: fix parameter checking

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>