drm/amd/display: Fix incorrect cursor pos on scaled primary plane

[Why]
Cursor pos is correctly adjusted from DC side for source rect offset
on DCN ASIC, but only on the overlay.

This is because DM places offsets the cursor for primary planes only
to workaround missing code in DCE for the adjustment we're now correctly
doing in DC for DCN ASIC.

[How]
Drop the adjustment for source rect from the DM side of things and put
the code where it actually belongs - in DC on the pipe level.

This matches what we do for DCN now.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Zhan Liu <Zhan.Liu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>