tgsi: split tgsi_util_get_texture_coord_dim() function into two

tgsi: split tgsi_util_get_texture_coord_dim() function into two

It was kind of overloaded, returning two different things.  Now get
the index of the shadow reference src register with a new
tgsi_util_get_shadow_ref_src_index() function.

To verify the new code, I added some temp/debug code which looped
over all TGSI_TEXTURE_x values, calling the old function and new and
checking that the returned indexes matched.

Also tested piglit "shadow" tests with softpipe/llvmpipe.
No testing of ilo and radeonsi changes.

Reviewed-by: Dave Airlie <airlied@redhat.com>