drm/amd/display/amdgpu_dm: Fix backlight_device_register() error handling

backlight_device_register() returns an ERR_PTR on error, but other code
such as amdgpu_dm_connector_destroy() assumes dm->backlight_dev[i] is NULL
if no backlight is registered.

Clear dm->backlight_dev[i] on registration failure, to avoid other code
trying to deref an ERR_PTR pointer.
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -4218,9 +4218,10 @@ amdgpu_dm_register_backlight_device(struct amdgpu_display_manager *dm)
 								       &amdgpu_dm_backlight_ops,
 								       &props);
 
-	if (IS_ERR(dm->backlight_dev[dm->num_of_edps]))
+	if (IS_ERR(dm->backlight_dev[dm->num_of_edps])) {
 		DRM_ERROR("DM: Backlight registration failed!\n");
-	else
+		dm->backlight_dev[dm->num_of_edps] = NULL;
+	} else
 		DRM_DEBUG_DRIVER("DM: Registered Backlight device: %s\n", bl_name);
 }