drm/amd/display: allocate 4 ddc engines for RV2

[Why]
Driver will create 0, 1, and 2 ddc engines for RV2,
but some platforms used 0, 1, and 3.

[How]
Still allocate 4 ddc engines for RV2.

Signed-off-by: Derek Lai <Derek.Lai@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>