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: 9e869063 ("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>