tgsi: add tgsi_util_get_texture_coord_dim()

tgsi: add tgsi_util_get_texture_coord_dim()

This util function returns the dimension of the texture coordinates for a
texture target, and the location of the shadow reference value.

For example, when the texture target is TGSI_TEXTURE_SHADOW2D, the dimension
of the texture coordinates is 2, and the location of the ref value is 2
(that is, the Z channel).

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