drm/amdkfd: add proper handling for S0ix

When in S0i3, the GFX state is retained, so all we need to do
is stop the runlist so GFX can enter gfxoff.

Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Tested-by: David Perry <david.perry@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 4bfa8609)
Cc: stable@vger.kernel.org