drm/i915: Make intel_dp_unpack_aux() static

This was introduced in:

  commit 0bc12bcb
  Author: Rodrigo Vivi <rodrigo.vivi@intel.com>
  Date:   Fri Nov 14 08:52:28 2014 -0800

      drm/i915: Introduce intel_psr.c

But the unpack function is unused at this date.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>