drm/amd/display: add dummy pstate workaround to dcn315

DCN315 has to always allow pstate change or SMU will hang. This
workaround achieves this by applying a low pstate change latency
to be used when pstate is calculated to be unsupported. This lower
latency only accounts for memory retraining; a previous change
handles locking in the highest available pstate allowing us to minimize
required latency hiding to only account for memory retraining.

Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>