drm/amd/display: Correct z8 watermark mask

Correct z8_watermark mask from 16bit to 20bit. Also, do not set dcn35
dprefclk in clk_mgr_construct.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Muhammad Ahmed <ahmed.ahmed@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>