drm/amd/display: Fix context copy memory leak

This change corrects an error introduced in 355f123f. Instead of
using the copy constructor to assign the new context, we swap the
pointer.

Signed-off-by: Corbin McElhanney <corbin.mcelhanney@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>