drm/amdgpu/gmc10: don't touch gfxhub registers during S0ix

gfxhub registers are part of gfx IP and should not need to be
changed.  Doing so without disabling gfxoff can hang the gfx IP.

v2: add comments explaining why we can skip the interrupt
    control for S0i3

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