drm/amdgpu: fix VRAM partially encroached issue in GDDR6 memory training(V2)

[why]
In GDDR6 BIST training, a certain mount of bottom VRAM will be encroached by
UMC, that causes problems(like GTT corrupted and page fault observed).

[how]
Saving the content of this bottom VRAM to system memory before training, and
restoring it after training to avoid VRAM corruption.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Tianci.Yin <tianci.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>