drm/i915: Remove unused WATCH_GTT define

Chris removed the code using it in:

  commit be2d599b
  Author: Chris Wilson <chris@chris-wilson.co.uk>
  Date:   Wed Sep 10 19:52:18 2014 +0100

      drm/i915: Remove dead code, i915_gem_verify_gtt

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