Documentation/gpu: Document amdgpu_dm_visual_confirm debugfs entry

Display core provides a feature that makes it easy for users to debug
Multiple planes by enabling a visual notification at the bottom of each
plane. This commit introduces how to use such a feature.

Reviewed-by: Yann Dirson <ydirson@free.fr>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>