drm: selftest: convert drm_dp_mst_helper selftest to KUnit

Considering the current adoption of the KUnit framework, convert the
DRM DP MST helper selftest to the KUnit API.

Co-developed-by: Rubens Gomes Neto <rubens.gomes.neto@usp.br>
Signed-off-by: Rubens Gomes Neto <rubens.gomes.neto@usp.br>
Tested-by: David Gow <davidgow@google.com>
Acked-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Maíra Canal <maira.canal@usp.br>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220708203052.236290-7-maira.canal@usp.br