drm/amd/display: Increase Z8 watermark times.

Increase Z8 watermark times from 210->250us and 320->350us.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Natanel Roizenman <natanel.roizenman@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>