drm/amd/display: Use dummy pstate latency for subvp when needed on dcn32

[WHY?]
Prefetch is not budgetting time for dummy pstate when using subvp and low uclk.

[HOW?]
Override fclk change latency to use dummy pstate latency when calculating
prefetch schedule for subvp configs with low uclk.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Dillon Varone <Dillon.Varone@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>