drm/amd/display: update pixel rate div in enable stream

[why]
Pixel rate div depends on the type of encoder
that we are enabling stream with. If we swap between
HPO and DIO encoder at the time we call enable stream
for the new encoder, we must reprogram pixel rate div
based on the new encoder type.

Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>