drm/amdgpu: add DMUB trace event IRQ source define

[Why & How]
We use DMCUB outbox0 interrupt to log DMCUB trace buffer events
as Linux kernel traces, so need to add some irq source related
defination in the header files;

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Leo (Hanghong) Ma <hanghong.ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>