drm/amdgpu: move mes self test after drm sched re-started

mes self test rely on vm mapping, move it after
drm sched re-started so that vm mapping can work
during gpu reset.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Acked-and-tested-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>