drm/amd/display: correct z8_watermark 16bit to 20bit mask

remove double adjustment for DPREFCLK SS. dprefclk adjusted with SS is
used for dp audio only. if adjust DP_DTO, need to adjust VID_M/N

Reviewed-by: Muhammad Ahmed <ahmed.ahmed@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>