i915: Only emit program errors when INTEL_DEBUG=wm or INTEL_DEBUG=fallbacks

i915: Only emit program errors when INTEL_DEBUG=wm or INTEL_DEBUG=fallbacks

This makes piglit a lot more happy.  The errors are logged when
INTEL_DEBUG=fallbacks because the application is about to hit a big
software fallback.  We frequently ask people to run applications that
are hitting software fallbacks with INTEL_DEBUG=fallbacks so the we
can help them debug the reason for the software fallback.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>