drm/amd/display: Force flush after write to IPS driver signals

[Why]
It's possible that the write hasn't fully completed by the time we
send (and flush) a command to DMCUB to notify idle to request IPS2
exit.

[How]
Perform a readback of the volatile structure into dc_dmub_srv state.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>