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>