drm/amd/display: Keep VBios pixel rate div setting until next mode set

drm/amd/display: Keep VBios pixel rate div setting until next mode set

[why]
Vbios & Driver have difference pixel rate div policy.
When enabling fast boot & performing blank & unblank w/o timing setting,
pixel clock & pixel rate dividor are not match.
It would cause too high pixel reate and eDP would be black screen.

[How]
We would keep pixel rate div setting by Vbios until next timing setting.

Reviewed-by: Jun Lei <jun.lei@amd.com>
Acked-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: yi-lchen <yi-lchen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>