drm/amd/display: limit the code change to ips enabled asic

Limit the code change for ips enable to reduce the impact for now. Also
exit_ips first before dc_power_down to avoid 0x9f.

Reviewed-by: Chris Park <chris.park@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>