drm/amd/display: Do not fill Null packet in the blank period

[Description]
Do not fill Null packet in the blank period for new packet gen
This is based on HW IP team recommended default setting change.

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>