drm/amdgpu: rename direct to immediate for VM updates

To avoid confusion with direct ring submissions rename bottom
of pipe VM table changes to immediate 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>