drm/amd/display: Introduce KUnit to DML

KUnit unifies the test structure and provides helper tools that simplify
the development of tests. Basic use case allows running tests as regular
processes, which makes easier to run unit tests on a development machine
and to integrate the tests in a CI system.

This commit introduces a basic unit test to one part of the Display Mode
Library: display_mode_lib, in order to introduce the basic structure of the
tests on the DML.

Signed-off-by: Maíra Canal <maira.canal@usp.br>