anv: Add error handling to setup_empty_execbuf().

The anv_execbuf_add_bo() call can actually fail in practice, which
should cause the QueueSubmit operation to fail.  Reported by Coverity.

CID: 1416606: Unchecked return value (CHECKED_RETURN)
Fixes: 017cdb10cf (anv: Submit a dummy batch when only semaphores are provided.)
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
1 file changed