drm/i915: Introduce i915_gem_set_seqno()

This function can be used to set the driver's next_seqno
to arbitrary value.

i915_gem_set_seqno() will idle the gpu, retire outstanding
requests, clear the semaphore mailboxes and set the hardware
status page's seqno index.

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
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 514aee8..b0faa91 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1479,8 +1479,8 @@
 	return (int32_t)(seq1 - seq2) >= 0;
 }
 
-extern int i915_gem_get_seqno(struct drm_device *dev, u32 *seqno);
-
+int __must_check i915_gem_get_seqno(struct drm_device *dev, u32 *seqno);
+int __must_check i915_gem_set_seqno(struct drm_device *dev, u32 seqno);
 int __must_check i915_gem_object_get_fence(struct drm_i915_gem_object *obj);
 int __must_check i915_gem_object_put_fence(struct drm_i915_gem_object *obj);