drm/amd/display: Add documentation and cleanup DMUB HW lock manager

drm/amd/display: Add documentation and cleanup DMUB HW lock manager

To document how the helpers should be used before a subsequent change
adds new locations that these helpers will be used.

Reviewed-by: Ovidiu (Ovi) Bunea <ovidiu.bunea@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>