drm/amd/display: Enable Z10 flag for IPS FSM

drm/amd/display: Enable Z10 flag for IPS FSM

[why]
IPS FSM requires Z10 flag to be enabled to do save and restore the
registers properly.

[how]
Enable Z10 and use the correct function to determine Z10 capability

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Sung Joon Kim <sungjoon.kim@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>