drm/amd/display: Fix brightness level not retained over reboot

[Why]
During boot up and resume the DC layer will reset the panel
brightness to fix a flicker issue.

It will cause the dm->actual_brightness is not the current panel
brightness level. (the dm->brightness is the correct panel level)

[How]
Set the backlight level after do the set mode.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Fixes: d9e8658 ("drm/amd/display: Simplify brightness initialization")
Reported-by: Mark Herbert <mark.herbert42@gmail.com>
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3655
Reviewed-by: Sun peng Li <sunpeng.li@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Author: Tom Chung

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -9442,6 +9442,7 @@ static void amdgpu_dm_commit_streams(struct drm_atomic_state *state,
 	bool mode_set_reset_required = false;
 	u32 i;
 	struct dc_commit_streams_params params = {dc_state->streams, dc_state->stream_count};
+	bool set_backlight_level = false;
 
 	/* Disable writeback */
 	for_each_old_connector_in_state(state, connector, old_con_state, i) {
@@ -9561,6 +9562,7 @@ static void amdgpu_dm_commit_streams(struct drm_atomic_state *state,
 			acrtc->hw_mode = new_crtc_state->mode;
 			crtc->hwmode = new_crtc_state->mode;
 			mode_set_reset_required = true;
+			set_backlight_level = true;
 		} else if (modereset_required(new_crtc_state)) {
 			drm_dbg_atomic(dev,
 				       "Atomic commit: RESET. crtc id %d:[%p]\n",
@@ -9612,6 +9614,19 @@ static void amdgpu_dm_commit_streams(struct drm_atomic_state *state,
 				acrtc->otg_inst = status->primary_otg_inst;
 		}
 	}
+
+	/* During boot up and resume the DC layer will reset the panel brightness
+	 * to fix a flicker issue.
+	 * It will cause the dm->actual_brightness is not the current panel brightness
+	 * level. (the dm->brightness is the correct panel level)
+	 * So we set the backlight level with dm->brightness value after set mode
+	 */
+	if (set_backlight_level) {
+		for (i = 0; i < dm->num_of_edps; i++) {
+			if (dm->backlight_dev[i])
+				amdgpu_dm_backlight_set_level(dm, i, dm->brightness[i]);
+		}
+	}
 }
 
 static void dm_set_writeback(struct amdgpu_display_manager *dm,