drm/amd/display: Adding debug option to override Z8 watermark values

[Why & How]
Adding debug options to override Z8 watermark values for testing purposes.

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Leo Chen <sancchen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>