r600g: disable shader rebuild optimization & account cb flush packet

r600g: disable shader rebuild optimization & account cb flush packet

Shader rebuild should be more clever, we should store along each
shader all the value that change shader program rather than using
flags in context (ie change sequence like : change vs buffer, draw,
change vs buffer, switch shader will trigger useless shader rebuild).

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