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

The bw_fixed library performs a lot of the mathematical operations
involving fixed-point arithmetic and the conversion of integers to
fixed-point representation.

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.

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