drm/amd/display: Add comment explaining context free

This comment explains a previous change that adds some complexity
to the context free in dc:

commit 9ad58779a895 ("drm/amd/display: Fix accessing freed memory")

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