drm/i915: Make intel_logical_ring_advance_and_submit() static

This function is only used in intel_lrc.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>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>