drm/amd/display: Fix wrong return value in dm_update_plane_state()

On an error exit path, a negative error code should be returned
instead of a positive return value.

Fixes: 9e869063b0 ("drm/amd/display: Move iteration out of dm_update_planes")
Cc: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -8343,8 +8343,7 @@ static int dm_update_plane_state(struct dc *dc,
 				dm_old_plane_state->dc_state,
 				dm_state->context)) {
 
-			ret = EINVAL;
-			return ret;
+			return -EINVAL;
 		}