drm/amd/display: Add DCN42 PMO policy for DML2.1

drm/amd/display: Add DCN42 PMO policy for DML2.1

[Why]
The MinTTU policy in DML2.1 does not guarantee that we support p-state
in blank. This is a delta vs dml2 and earlier revisions as the prefetch
mode override has been removed in favor of a more configurable pstate
optimizer.

[How]
Split off DCN42 with its own PMO helpers so that we can use a simpler
strategy of only allowing the mode if we support p-state in vblank and
if vactive has enough latency hiding.

The actual hookup to use these helpers in the PMO factory will be
done in a later patch to satisfy build system requirements.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>