drm/amdgpu: Remove unused dc_stream from amdgpu_crtc

It's no longer used. In fact, there is no more dc_stream object.

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>