tgsi: add TXF support.

tgsi: add TXF support.

This is a straight texel fetch with no filtering or clamping. It uses
integers to specify the i/j/k (from EXT_gpu_shader4).

To enable this I had to add another hook into the tgsi sampler so that
we could easily bypass all the filtering sample does.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Brian Paul <brianp@vmware.com>