iris: Spruce up "are we using this engine?" checks for flushing

We were using batch->contains_draw as a proxy for "are we even using
this engine?"  That isn't quite right, because it only counts regular
draws.  BLORP operations may have also rendered to a resource, which
needs to trigger flushing.  To check for this, we also see if the
render and sometimes depth caches are non-empty.

We can also drop the "but there might already be stale data in the
cache even if we haven't emitted any commands yet" concern in the
comments.  The kernel flushes caches between batches.

This may not be great but it's at least better than what was there.

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