drm/amd/display: Add DMUB tracebuffer debugfs

[Why]
The DMUB tracebuffer is useful for understanding DMCUB execution state.

[How]
Add a "show" attribute debugfs so we can loop through the buffer
and print the entries.

The structs for the entry format are defined in the debugfs since
the tracebuffer header no longer exists in the DMUB service.

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