drm/amd/display: Use DTO as clock on DP if not

Use DVO as pixel clock for DP before enabling link
PHY. Otherwise, when switching from HDMI passive dongle
to DP on the same connector, the PHY PLL is used as
pixel clock, and CRTC would stop working.

Signed-off-by: Ding Wang <Ding.Wang@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>