drm/amd/display: Fix DCN42B null registers & register masks

drm/amd/display: Fix DCN42B null registers & register masks

[why]

DCN42B is missing some register masks, which are causing errors in dmesg.

[how]

Make DCN42B reuse the DCN42 register lists, and add the missing defines manually.

Fixes: 64142f9d51af ("drm/amd/display: Fix DCN42 null registers & register masks")
Reviewed-by: Ovidiu (Ovi) Bunea <ovidiu.bunea@amd.com>
Signed-off-by: Matthew Stewart <Matthew.Stewart2@amd.com>
Signed-off-by: George Zhang <george.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>