drm/amd/display: Allow higher DSC slice support for small timings on dcn401

[WHY]
DML2.1 has added the support to determine ODM combine based on DSC slice
count limitation. This support would allow us to support DSC slice higher
than 4 on small timings. The change will allow higher DSC slice support
independent from pixel clock in use.

[HOW]
Add a DCN401 get_enc_caps function to allow the support for DSC slice count
higher than 4.
Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>