drm/amdgpu: Check extended configuration space register when system uses large bar

Some customer platforms do not enable mmconfig for various reasons,
such as bios bug, and therefore cannot access the GPU extend configuration
space through mmio.

When the system enters the d3cold state and resumes, the amdgpu driver
fails to resume because the extend configuration space registers of
GPU can't be restored. At this point, Usually we only see some failure
dmesg log printed by amdgpu driver, it is difficult to find the root
cause.

Therefor print a warnning message if the system can't access the
extended configuration space register when using large bar.

Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>