drm/amd/display: Introduce KUnit tests for fixed31_32 library

The fixed31_32 library performs a lot of the mathematical operations
involving fixed-point arithmetic and the conversion of integers to
fixed-point representation.

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. Use kunit_tool to run:

$ ./tools/testing/kunit/kunit.py run --arch=x86_64 \
	--kunitconfig=drivers/gpu/drm/amd/display/tests/

Signed-off-by: Tales Aparecida <tales.aparecida@gmail.com>