drm/amd/display: Use drm_connector in create_stream_for_sink

[WHAT]
We need to use this function for both amdgpu_dm_connectors
and drm_writeback_connectors. Modify it to operate on
a drm_connector as a common base.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>