drm/amdgpu: don't mess with SDMA clock or powergating in S0ix

It's handled by GFXOFF for SDMA 5.x and SMU saves the state on
SDMA 4.x.

Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>