drm/amd/display: Introduce KUnit tests to the bw_fixed library

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 unit test to the bw_fixed library, which
performs a lot of the mathematical operations involving fixed-point
arithmetic and the conversion of integers to fixed-point representation
inside the Display Mode Library.

As fixed-point representation is the base foundation of the DML calcs
operations, this unit tests intend to assure the proper functioning of
the basic mathematical operations of fixed-point arithmetic, such as
multiplication, conversion from fractional to fixed-point number, and
more.  You can run it with: ./tools/testing/kunit/kunit.py run \
	--arch=x86_64 \
	--kunitconfig=drivers/gpu/drm/amd/display/tests/

Signed-off-by: Maíra Canal <maira.canal@usp.br>
Co-developed-by: Magali Lemes <magalilemes00@gmail.com>
Signed-off-by: Magali Lemes <magalilemes00@gmail.com>
Co-developed-by: Tales Aparecida <tales.aparecida@gmail.com>
Signed-off-by: Tales Aparecida <tales.aparecida@gmail.com>