drm/amd/display: Fix DP PHY test pre-emphasis not set properly

Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>