drm/amd/display: fix hibernate entry for DCN35+

Since, two suspend-resume cycles are required to enter hibernate and,
since we only need to enable idle optimizations in the first cycle
(which is pretty much equivalent to s2idle). We can check in_s0ix, to
prevent the system from entering idle optimizations before it actually
enters hibernate (from display's perspective). Also, call
dc_set_power_state() before dc_allow_idle_optimizations(), since it's
safer to do so because dc_set_power_state() writes to DMUB.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>