drm/amdgpu: invalidate only the currently needed VMHUB v2

Drop invalidating both hubs from each engine.

v2: don't use hardcoded values

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Andres Rodriguez <andresx7@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>