drm/amd/display: Add support for hw_state logging via debugfs

[Why]

We have logging methods for printing hardware state for newer ASICs
but no way to trigger the log output.

[How]

Add support for triggering the output via writing to a debugfs file
entry. Log output currently goes into dmesg for convenience, but
accessing via a read should be possible later.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Jordan Lazare <Jordan.Lazare@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>