drm/amd/display: Restore correct backlight brightness after a GPU reset

[Why]
GPU reset will attempt to restore cached state, but brightness doesn't
get restored. It will come back at 100% brightness, but userspace thinks
it's the previous value.

[How]
When running resume sequence if GPU is in reset restore brightness
to previous value.

Acked-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Author: superm1

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

@@ -250,6 +250,10 @@ static int amdgpu_dm_atomic_check(struct drm_device *dev,
 static void handle_hpd_irq_helper(struct amdgpu_dm_connector *aconnector);
 static void handle_hpd_rx_irq(void *param);
 
+static void amdgpu_dm_backlight_set_level(struct amdgpu_display_manager *dm,
+					 int bl_idx,
+					 u32 user_brightness);
+
 static bool
 is_timing_unchanged_for_freesync(struct drm_crtc_state *old_crtc_state,
 				 struct drm_crtc_state *new_crtc_state);
@@ -3432,6 +3436,12 @@ static int dm_resume(struct amdgpu_ip_block *ip_block)
 
 		mutex_unlock(&dm->dc_lock);
 
+		/* set the backlight after a reset */
+		for (i = 0; i < dm->num_of_edps; i++) {
+			if (dm->backlight_dev[i])
+				amdgpu_dm_backlight_set_level(dm, i, dm->brightness[i]);
+		}
+
 		return 0;
 	}