drm/amd/display: Fix dummy p-state hang on monitors with extreme timing

[WHY]
It was found that the system would hang on a dummy pstate when playing
4k60 videos on a 1080p 390Hz monitor.

[HOW]
Properly select the dummy_pstate_latency_ms when firmware assisted
memory clock switching is enabled instead of assuming that the highest
latency would work for every monitor timing.

Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Felipe Clark <felclark@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>