drm/amdgpu: add mcbp unit test in debugfs (v3)

The MCBP unit test is used to test the functionality of MCBP.
It emualtes to send preemption request and resubmit the unfinished
jobs.

v2: squash in fixes (Alex)
v3: squash in memory leak fix (Jack)

Acked-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>