drm/amdgpu: update comments about s0ix suspend/resume

Provide and explanation as to why we skip GFX and PSP for
S0ix.  GFX goes into gfxoff, same as runtime, so no need
to tear down and re-init.  PSP is part of the always on
state, so no need to touch it.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>