i915: use _mesa_delete_buffer_object

This is more future-proof, plugs the memory leak of Label and properly
destroys the buffer mutex.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Cc: "11.0 11.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
(cherry picked from commit 1b74c02e83)

src/mesa/drivers/dri/i915/intel_buffer_objects.c

@@ -99,7 +99,7 @@ intel_bufferobj_free(struct gl_context * ctx, struct gl_buffer_object *obj)
    _mesa_align_free(intel_obj->sys_buffer);
 
    drm_intel_bo_unreference(intel_obj->buffer);
-   free(intel_obj);
+   _mesa_delete_buffer_object(ctx, obj);
 }