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>

drivers/gpu/drm/amd/display/dc/dc_dmub_srv.c

@@ -1296,6 +1296,7 @@ static void dc_dmub_srv_notify_idle(const struct dc *dc, bool allow_idle)
 		}
 
 		ips_driver->signals = new_signals;
+		dc_dmub_srv->driver_signals = ips_driver->signals;
 	}
 
 	DC_LOG_IPS(
@@ -1339,6 +1340,7 @@ static void dc_dmub_srv_exit_low_power_state(const struct dc *dc)
 		ips2_exit_count = ips_fw->ips2_exit_count;
 
 		ips_driver->signals.all = 0;
+		dc_dmub_srv->driver_signals = ips_driver->signals;
 
 		DC_LOG_IPS(
 			"%s (allow ips1=%d ips2=%d) (commit ips1=%d ips2=%d) (count rcg=%d ips1=%d ips2=%d)",

drivers/gpu/drm/amd/display/dc/dc_dmub_srv.h

@@ -53,6 +53,7 @@ struct dc_dmub_srv {
 	void *dm;
 
 	int32_t idle_exit_counter;
+	union dmub_shared_state_ips_driver_signals driver_signals;
 	bool idle_allowed;
 	bool needs_idle_wake;
 };