drm/amd/display: Fix wrong index for DCN401 cursor offload

drm/amd/display: Fix wrong index for DCN401 cursor offload

[Why]
Payloads are ignored because the wrong index is written as part of the
pipe update implementation for DCN401.

[How]
Align it to the DCN35 implementation and ensure the + 1 is added.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>