drm/amd/display: Use swap() to simplify code

Replace the original swapping logic with swap() to improve readability and
remove temporary variables

Signed-off-by: Liao Yuanhong <liaoyuanhong@vivo.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>