drm/amd/display: Assign disp_cfg_index_max when dml21

[WHY & HOW]
The assignment of disp_cfg_index_max is missed and should be
assigned to __DML2_WRAPPER_MAX_STREAMS_PLANES__.

Fixes: 55ec7679 ("drm/amd/display: Limit array index according to architecture")
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>