drm/amd/display: Use correct pixel clock to program DTBCLK DTO's

[Why?]
Currently phy_pix_clk is used to program DTO's which is incorrect.

[How?]
Use the timing pixel clock to program DTO's correctly.

Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Dillon Varone <Dillon.Varone@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>