drm/amd/display: Fix black screen when disabling Freesync in OSD

[Why]
Black screen encountered when disabling Freesync through OSD on some
displays.

[How]
Set the should_disable flag when new top pipe has no plane state to
ensure that pipes get cleaned up.

Reviewed-by: Chris Park <Chris.Park@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Ilya Bakoulin <Ilya.Bakoulin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>