drm/i915: kill intel_crtc_update_sarea_pos

We assign the sarea_priv pointer only in the dma ioctl, which is
disallowed when kernel modesetting is enabled. So this is dead code.

Cc: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>