drm/i915: Track purged state.

In order to correctly prevent the invalid reuse of a purged buffer, we
need to track such events and warn the user before something bad
happens.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

include/drm/i915_drm.h

@@ -671,6 +671,7 @@ struct drm_i915_get_pipe_from_crtc_id {
 
 #define I915_MADV_WILLNEED 0
 #define I915_MADV_DONTNEED 1
+#define __I915_MADV_PURGED 2 /* internal state */
 
 struct drm_i915_gem_madvise {
 	/** Handle of the buffer to change the backing store advice */