drm/amdgpu: Add DMCUB to firmware query interface

The DMCUB firmware version can be read using the AMDGPU_INFO ioctl
or the amdgpu_firmware_info debugfs entry.

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>