drm/i915: Remove assertion over write domain after i915_gem_object_sync()

As we move to lazily clearing the GPU write domain only when the buffer
becomes inactive, this leaves a window of opportunity for
i915_gem_object_pin_to_display_plane() to detect a seemingly
inconsistent value. This function is special as it tries to pipeline the
operation to avoid the stall and so may not retires the buffer and we
may not get the opportunity to clear the write domain. However, we know
all is good, so drop the assertion.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -3091,7 +3091,7 @@ i915_gem_object_pin_to_display_plane(struct drm_i915_gem_object *obj,
 	/* It should now be out of any other write domains, and we can update
 	 * the domain values for our changes.
 	 */
-	BUG_ON((obj->base.write_domain & ~I915_GEM_DOMAIN_GTT) != 0);
+	obj->base.write_domain = 0;
 	obj->base.read_domains |= I915_GEM_DOMAIN_GTT;
 
 	trace_i915_gem_object_change_domain(obj,