Documentation/gpu: Remove ':export:' option from DCN documentation

This commit reduces, but does not fix, all the occurrences and some of
the documentation warnings related to the 'no structured comments.' This
was caused by the wrong use of the ':export:' option in the DCN
kernel-doc, so this commit drops the usage of those options.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>