drm/amdgpu: reserve vram for memory training(v4)

memory training using specific fixed vram segment, reserve these
segments before anyone may allocate it.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Tianci.Yin <tianci.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>