drm/amd/display: Add simple struct doc to remove doc build warning

This commit is a part of a series that addresses the following build
warning for opp:

./drivers/gpu/drm/amd/display/dc/inc/hw/opp.h:1: warning: no structured
comments found
./drivers/gpu/drm/amd/display/dc/inc/hw/dpp.h:1: warning: no structured
comments found

This commit fixes this issue by adding a simple kernel-doc to a struct
in the opp.h and the dpp.h files.

Cc: Alex Deucher <alexander.deucher@amd.com>
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>