drm/amdgpu: add dmi check for jadeite platform

DMI check is required to distinguish Jadeite platform from
Stoney base variant.
Add DMI check logic for Jadeite platform.

Signed-off-by: Vijendar Mukunda <Vijendar.Mukunda@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>