mesa: assert if driver did not compute the version

mesa: assert if driver did not compute the version

Make sure drivers initialize the version before:
 * _mesa_initialize_exec_table is called
 * _mesa_initialize_exec_table_vbo is called
 * A context is made current

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>