drm/amd/display/dc/core/dc_resource: use swap macro in rect_swap_helper

Make use of the swap macro instead of _manually_ swapping values
and remove unnecessary variable temp.

This makes the code easier to read and maintain.

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>