drm/i915: Make intel_dp_check_link_status() static

This function is only used in intel_dp.c since:

  commit 0e32b39c
  Author: Dave Airlie <airlied@redhat.com>
  Date:   Fri May 2 14:02:48 2014 +1000

      drm/i915: add DP 1.2 MST support (v0.7)

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