drm/amd/display: support "dummy pstate"

drm/amd/display: support "dummy pstate"

[why]
Existing support in DC for pstate only accounts for a single latency.  This is sufficient when the
variance of latency is small, or that pstate support isn't necessary for correct ASIC functionality.

Newer ASICs violate both existing assumptions.  PState support is mandatory of correct ASIC
functionality, but not all latencies have to be supported.  Existing code supports a "full p state" which
allows memory clock to change, but is hard for DCN to support (as it requires very large buffers).
New code will now fall back to a "dummy p state" support when "full p state" cannot be support.
This easy p state support should always be allowed.

[how]
Define a new latency in socBB.  Add fallback logic to support it.  Note DML is also updated to ensure
that fallback will always work.

Signed-off-by: Jun Lei <Jun.Lei@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>