amdgpu: Add Missing Sienna Cichlid DID

The purpose of this patch is to add a missing device ID for Sienna Cichlid.
The missing ID "0x73A1" is now added to the "amdgpu_drv.c" file.

Signed-off-by: Ori Messinger <Ori.Messinger@amd.com>
Reviewed-by: Kent Russell <kent.russell@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>