tgsi: clean up exec_tex()

tgsi: clean up exec_tex()

Make use of tgsi_util_get_texture_coord_dim() to replace the big switch table.

There is a subtle difference with this change.  When TXP is used with an array
texture, the layer is now also projected.  This behavior matches the TGSI doc.
Since GLSL does not allow TXP on an array texture, I am not sure which
behavior is correct or preferred.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Acked-by: Roland Scheidegger <sroland@vmware.com>