[Why]
We want to log DMCUB trace buffer events as Linux kernel traces.

[How]
Register an IRQ handler for DMCUB outbox0 interrupt in amdgpu_dm,
and log the messages in the DMCUB tracebuffer to a new DMCUB
TRACE_EVENT as soon as we receive the outbox0 IRQ from DMCUB FW.
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Leo (Hanghong) Ma <hanghong.ma@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>