drm/amdgpu: fix modprobe failure of the secondary GPU when GDDR6 training enabled(V5)

[why]
In dual GPUs scenario, stolen_size is assigned to zero on the secondary GPU,
since there is no pre-OS console using that memory. Then the bottom region of
VRAM was allocated as GTT, unfortunately a small region of bottom VRAM was
encroached by UMC firmware during GDDR6 BIST training, this cause page fault.

[how]
Forcing stolen_size to 3MB, then the bottom region of VRAM was
allocated as stolen memory, GTT corruption avoid.

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