drm/amd/display: increase Z9 latency to workaround underflow in Z9

[Why]
Z9 latency is higher than when we originally tuned the watermark
parameters, causing underflow. Increasing the value until the latency
issues is resolved.

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Agustin Gutierrez Sanchez <agustin.gutierrez@amd.com>
Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org