drm/amdgpu: mark the partial job as preempted in mcbp unit test

In mcbp unit test, the test should detect the preempted job which may
be a partial execution ib and mark it as preempted; so that the gfx
block can correctly generate PM4 frame.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>