drm/amd/display: Add DMUB firmware state debugfs

[Why]
Firmware state helps to debug sequence issues and hangs for DMCUB
commands and we don't have an easy mechanism to dump it from the driver.

[How]
Add a debugfs entry to dump the current firmware state.
Example usage:

cat /sys/kernel/debug/dri/0/amdgpu_dm_dmub_fw_state

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>