drm/radeon/kms: don't apply 7xx HDP flush workaround on AGP

It should be required for all 7xx asics, but seems to cause
problems on some AGP 7xx chips.

Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=19002



Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Reported-and-Tested-by: Duncan <1i5t5.duncan@cox.net>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>