Documentation/gpu: Add Display Core Unit Test documentation

Explain how to run the KUnit tests present in the AMDGPU's Display
Core and clarify which architectures and tools can be used to run
the tests. Moreover, explains how to add new tests to the existing
tests.

Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Tales Aparecida <tales.aparecida@gmail.com>