r600g: force unbind of previously bind sampler/sampler_view

r600g: force unbind of previously bind sampler/sampler_view

Previously bind sampler/sampler_view can be converted and endup
overwritting the current state we want to schedule. Example :
bind texA texB to sampler_view[0] & sampler_view[1], render,
bind texB to sampler_view[0] render. Now state associated to
texB are set to configure sampler_view slot 0, but as we don't
unbind sampler_view[1] still point to texB state so we end up
with sampler_view[1] overwritting sampler_view[0], which gives
wrong rendering if next rendering bind texA to sampler_view[0],
it will endup as texB is bound to sampler_view[0]. If you are
not confuse at that point give me a call i will be buying you
beer.

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