[Why]
Vbios uses preferred timing to turn on edp but OS could use other
timing. If change pixel clock when link active, there is unexpected
garbage on monitor.

[How]
Once pixel clock changed, the driver needs to disable stream.
Signed-off-by: Chiawen Huang <chiawen.huang@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>