drm/amd/display: Add Z8 allow states to z-state support list

drm/amd/display: Add Z8 allow states to z-state support list

[Why]
Even if we block Z9 based on crossover threshold it's possible to
allow for Z8.

[How]
There's support for this on DCN314, so update the support types to
include a z8 only and z8_z10 only state.

Update the decide_zstate_support function to allow for specifying
these modes based on the Z8 threshold.

DCN31 has z-state disabled, but still update the legacy code to
map z8_only = disallow and z10_z8_only = z10_only to keep the support
the same.

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