drm/radeon/kms: Fix R600/RV770 disable acceleration path

When acceleration doesn't work we should free associated memory
and stop GPU block responsible for hardware acceleration so we
don't waste resource or let think one component of the driver that
a GPU feature is working/running while it doesn't actualy work.

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