drm/amd/display: Introduce KUnit tests for dcn20_fpu

This commit adds unit tests to the functions dcn20_cap_soc_clocks and
dcn21_update_bw_bounding_box from dcn20/dcn20_fpu.

Signed-off-by: Magali Lemes <magalilemes00@gmail.com>
Signed-off-by: Tales Aparecida <tales.aparecida@gmail.com>