etna_bo_del: make sure that FREE_VIDEO_MEMORY is sent as event

Right now the kernel will complain when etna_bo_del
is called without a queue and the unlock is delayed (async=1 is
returned), as the free happens immediately.

So make sure that FREE_VIDEO_MEMORY is sent as event even when
no queue is passed.
Call gcvHAL_DETACH from etna_free()

Make sure that we tell the kernel to clean up the HAL context
before we close the connection.