drm/i915: Extract intel_frontbuffer active tracking

Move the active tracking for the frontbuffer operations out of the
i915_gem_object and into its own first class (refcounted) object. In the
process of detangling, we switch from low level request tracking to the
easier i915_active -- with the plan that this avoids any potential
atomic callbacks as the frontbuffer tracking wishes to sleep as it
flushes.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190816074635.26062-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.h

@@ -2355,10 +2355,6 @@ int i915_gem_mmap_gtt(struct drm_file *file_priv, struct drm_device *dev,
 		      u32 handle, u64 *offset);
 int i915_gem_mmap_gtt_version(void);
 
-void i915_gem_track_fb(struct drm_i915_gem_object *old,
-		       struct drm_i915_gem_object *new,
-		       unsigned frontbuffer_bits);
-
 int __must_check i915_gem_set_global_seqno(struct drm_device *dev, u32 seqno);
 
 static inline u32 i915_reset_count(struct i915_gpu_error *error)