amdgpu/dm: Remove unused forward declaration

dc_stream has long been renamed to dc_stream_state, so this
forward declaration hasn't been used at all.
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h

@@ -199,8 +199,6 @@ struct amdgpu_framebuffer;
 struct amdgpu_display_manager;
 struct dc_validation_set;
 struct dc_plane_state;
-/* TODO rename to dc_stream_state */
-struct  dc_stream;
 
 struct dm_plane_state {
 	struct drm_plane_state base;