drm/amd/display: Use DTBCLK for valid pixel clock

Use DTBCLK for valid pixel clock generation

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Acked-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>