DC will use DMUB for command submission and flow control during
initialization.

Register offloading as well as submitting some BIOS commands are part
of the DC internal interface but are guarded behind debug options.

It won't be functional in amdgpu_dm yet since we don't pass the
DMUB service to DC for use.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>