mesa: Optimize unbind_texobj_from_texunits()

mesa: Optimize unbind_texobj_from_texunits()

The texture can only be bound to the index that corresponds to its
target, so there is no need to loop over all possible indices
for every unit and checking if the texture is bound to it.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>