drm/amd/display: Move DIO documentation to the right place

When building the kernel-doc, it complains with the below warning:

./drivers/gpu/drm/amd/display/dc/link/hwss/link_hwss_dio.h:1: warning: no structured comments found
./drivers/gpu/drm/amd/display/dc/link/hwss/link_hwss_dio.h:1: warning: no structured comments found

This warning was caused by the wrong use of the ':export:' and the lack
of function documentation in the file pointed under the ':internal:'.
This commit addresses those issues by relocating the overview
documentation to the correct C file, removing the ':export:' options,
and adding two simple kernel-doc to ensure that ':internal:' does not
have any warning.

Cc: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/dri-devel/20240715085918.68f5ecc9@canb.auug.org.au/


Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>