drm/radeon/evergreen/btc/fusion: setup hdp to invalidate and flush when asked

drm/radeon/evergreen/btc/fusion: setup hdp to invalidate and flush when asked

BugLink: http://bugs.launchpad.net/bugs/793702

commit f25a5c63bfa017498c9adecb24d649ae96ba5c68 upstream.

This needs to be explicitly set on btc.  It's set by default
on evergreen/fusion, so it fine to just unconditionally enable it for
all chips.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>