drm/i915: add i915_gem_context_get_hang_stats()

To get context hang statistics for specified context,
add i915_gem_context_get_hang_stats().

For arb-robustness, every context needs to have its own
hang statistics tracking. Added function will return
the user specified context statistics or in case of
default context, statistics from drm_i915_file_private.

v2: handle default context inside get_reset_state

v3: return struct pointer instead of passing it in as param
    (Chris Wilson)

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 124eb1a..cfa9286 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1812,6 +1812,10 @@
 	kref_put(&ctx->ref, i915_gem_context_free);
 }
 
+struct i915_ctx_hang_stats * __must_check
+i915_gem_context_get_hang_stats(struct intel_ring_buffer *ring,
+				struct drm_file *file,
+				u32 id);
 int i915_gem_context_create_ioctl(struct drm_device *dev, void *data,
 				  struct drm_file *file);
 int i915_gem_context_destroy_ioctl(struct drm_device *dev, void *data,