drm/amd/display: Add missing DCN21 IP parameter

[why]
IP parameter min_meta_chunk_size_bytes is read for bandwidth
calculations but it was never defined.

[how]
Define min_meta_chunk_size_bytes and initialize value to 256.

Reviewed-by: Laktyushkin Dmytro <dmytro.laktyushkin@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Victor Lu <victorchengchi.lu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>