drm/amd: Don't allow s0ix on APUs older than Raven

APUs before Raven didn't support s0ix.  As we just relieved some
of the safety checks for s0ix to improve power consumption on
APUs that support it but that are missing BIOS support a new
blind spot was introduced that a user could "try" to run s0ix.

Plug this hole so that if users try to run s0ix on anything older
than Raven it will just skip suspend of the GPU.

Fixes: cf488dcd ("drm/amd: Allow s0ix without BIOS support")
Suggested-by: Alexander Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>