drm/amd/display: Fix dcn21 num_states

[Why]
DML expects num_states to exclude the duplicate state.

[How]
Set num_states to correct value to prevent array off-by-one error.  Also
refactor max clock level code for diags.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>