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>

drivers/gpu/drm/i915/i915_drv.h

@@ -284,7 +284,6 @@ void intel_link_compute_m_n(int bpp, int nlanes,
 #define DRIVER_PATCHLEVEL	0
 
 #define WATCH_LISTS	0
-#define WATCH_GTT	0
 
 struct opregion_header;
 struct opregion_acpi;