drm/amd/display: Change default Z8 watermark values

[Why & How]
Previous Z8 watermark values were causing flickering and OTC underflow.
Updating Z8 watermark values based on the measurement.

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
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>