drm/amd/display/dc/dce/dce_opp: Demote non-compliant kernel-doc headers

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:109: warning: Function parameter or member 'opp110' not described in 'set_truncation'
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:109: warning: Function parameter or member 'params' not described in 'set_truncation'
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:154: warning: Function parameter or member 'opp110' not described in 'dce60_set_truncation'
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:154: warning: Function parameter or member 'params' not described in 'dce60_set_truncation'
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:204: warning: Function parameter or member 'opp110' not described in 'set_spatial_dither'
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:204: warning: Function parameter or member 'params' not described in 'set_spatial_dither'
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:308: warning: Function parameter or member 'opp110' not described in 'set_temporal_dither'
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:308: warning: Function parameter or member 'params' not described in 'set_temporal_dither'
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:370: warning: Function parameter or member 'opp110' not described in 'dce110_opp_set_clamping'
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:370: warning: Function parameter or member 'params' not described in 'dce110_opp_set_clamping'
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:430: warning: Function parameter or member 'opp110' not described in 'dce60_opp_set_clamping'
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:430: warning: Function parameter or member 'params' not described in 'dce60_opp_set_clamping'
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:478: warning: Function parameter or member 'opp110' not described in 'set_pixel_encoding'
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:478: warning: Function parameter or member 'params' not described in 'set_pixel_encoding'
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:514: warning: Function parameter or member 'opp110' not described in 'dce60_set_pixel_encoding'
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_opp.c:514: warning: Function parameter or member 'params' not described in 'dce60_set_pixel_encoding'

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Mauro Rossi <issor.oruam@gmail.com>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>