drm/amd/display: Remove unused dc_stream_get_crtc_position

The last user of dc_stream_get_crtc_position() was
mod_freesync_get_v_position() which is removed in a previous
patch in this series.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>