drm/amd/display: Fix potential access beyond end of array in CM

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
CC: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>