drm/amd/display: Move and Rename "is_stream_changed()"

 -Move "is_stream_changed()" to DC interface
 -Rename "is_stream_changed()" to "dc_is_stream_changed()"

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>