drm/amdgpu: Add ability to determine and report if board supports ECC.

Make initialization code check the ECC related registers, which are initialized
by the VBIOS, to see if ECC is present and initialized and DRM_INFO() the
result.

Signed-off-by: David Panariti <David.Panariti@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>