drm/amd/display: Add debugfs entry for amdgpu_dm_visual_confirm

drm/amd/display: Add debugfs entry for amdgpu_dm_visual_confirm

[Why]
DC provides a few visual confirmation debug options that can be
dynamically changed at runtime to help debug surface programming issues
but we don't have any way to access it from userspace.

[How]
Add the amdgpu_dm_visual_confirm debugfs entry.
It accepts a string containing the DC visual confirm enum value using
the debugfs attribute helpers.

The debugfs_create_file_unsafe can be used instead of
debugfs_create_file as per the documentation.

v2: Use debugfs helpers for getting and setting the value (Christian)

Cc: Leo Li <sunpeng.li@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>