drm/amd/display: Call ATOM_INIT instead of ATOM_ENABLE for DMCUB

[Why]
DMCUB command table doesn't support ATOM_ENABLE/ATOM_DISABLE anymore
so we never end up calling the DCN init path in DMCUB.

[How]
Map ATOM_ENABLE to ATOM_INIT only for DMCUB command table offloading.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>