drm/amdgpu: don't set s3 and s0ix at the same time

This makes it clearer which codepaths are in use specifically in
one state or the other.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>