VM support is only available for CZ and newer APUs. Trying to
enable it for dGPU will blow up in DC.

v2: Don't enable gpu_vm_support for Raven yet since it leads to
    a black screen. Need to debug this further before enabling.

Change-Id: Ibe467c36affe2e7a7ee740c8d4f73027ca807178
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Suggested-by: Alex Deucher <alexander.deucher@amd.com>
CC: Christian Koenig <christian.koenig@amd.com>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>