mesa, i965: prepare for more than 8 texture targets

mesa, i965: prepare for more than 8 texture targets

3-bit fields are used store texture target in several places.  That will fail
when TEXTURE_EXTERNAL_INDEX, which happends to be the 9th texture target, is
added.  Make them 4-bit fields.

Reviewed-by: Brian Paul <brianp@vmware.com>
Acked-by: Jakob Bornecrantz <jakob@vmware.com>