r600g: rework dirty / depth texture tracking.

r600g: rework dirty / depth texture tracking.

this adds a flag to keep track of whether the depth texture structure
is the flushed texture or not, so we can avoid doing flushes when
we do a hw rendering from one to the other.

it also renames flushed to dirty_db which tracks if the DB copy
has been dirtied by being bound to the hw.

Signed-off-by: Dave Airlie <airlied@redhat.com>