sna/gen6: Restore stall dropped when not flushing instead

sna/gen6: Restore stall dropped when not flushing instead

commit 82e6d41c2f4f343bd1854d3d8ee4b624b5d68971
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Thu Oct 31 13:35:59 2013 +0000

    sna/gen6: Tweak flush around CC state changes

Replaced the pipeline stall with a flush - but only when the target was
dirty. The missing stall however seems to be required as well.

v2: Actually emit the stall for all CC state changes [Ilia Mirkin]

Reported-by: Ilia Mirkin <imirkin@alum.mit.edu>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=72375
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>