drm/amdgpu: Add ucode support for DMCUB

The DMCUB is a secondary DMCU (Display MicroController Unit) that has
its own separate firmware. It's required for DMCU support on Renoir.

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>