i965/nir: Make our environment variable checking smarter

Before, we enabled NIR if you set INTEL_USE_NIR to anything which mean that
INTEL_USE_NIR=false would actually turn on NIR.  In preparation for turning
NIR on by default, this commit makes it smarter by allowing the
INTEL_USE_NIR variable to work as either a force-enable or a force-disable.

Reviewed-by: Mark Janes <mark.a.janes@intel.com>
1 file changed