st/mesa: provide the null implementation of bounding box outputs in tcs

Until hardware appears (in a gallium driver) that can make use of the
TCS-outputted gl_BoundingBox, we just request that the variable gets
assigned as a regular patch variable.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>