drm/i915: Make intel_ring_setup_status_page() static

This function is only used in intel_ringbuffer.c, so restrict it to that
file. The function was moved around to avoid a forward declaration and
group it with its user.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Squash in fixup from Wu Fengguang.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>