anv: Initialize cmd_buffer->state.current_db_mode for copy and video engines
What does this MR do and why?
Copy and video engines don't support STATE_BASE_ADDRESS, so current_db_mode was being left set to ANV_CMD_DESCRIPTOR_BUFFER_MODE_UNKNOWN causing it to trigger asserts in anv_cmd_buffer_alloc_dynamic_state(), get_cps_state_offset() and flush_descriptor_buffers().
So here initializing it ANV_CMD_DESCRIPTOR_BUFFER_MODE_LEGACY for copy and video engines.