drm/amd/display/dc/dce120/dce120_timing_generator: Demote non-kerneldoc headers

drm/amd/display/dc/dce120/dce120_timing_generator: Demote non-kerneldoc headers

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

 drivers/gpu/drm/amd/amdgpu/../display/dc/dce120/dce120_timing_generator.c:602:13: warning: ‘dce120_timing_generator_get_position’ defined but not used [-Wunused-function]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce120/dce120_timing_generator.c:73: warning: Cannot understand  *****************************************************************************
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce120/dce120_timing_generator.c:593: warning: Cannot understand  *****************************************************************************

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: 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>