drm/amd/display: Add disclaimer to BW and DML code provided by HW

This code can sometimes look troubling but we trust it as it comes from
HW teams with a guarantee of correctness. Add a note to these files to
explain this.

v2: thing -> things

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>