drm/amd/display: Make driver backwards-compatible with non-IPS PMFW

[why]
Driver needs to be compatible with PM FW that doesn't support IPS

[how]
Toggle internal control flag

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Ovidiu Bunea <ovidiu.bunea@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>