i965: Make perf_debug() output to GL_ARB_debug_output in a debug context.

i965: Make perf_debug() output to GL_ARB_debug_output in a debug context.

I tried to ensure that performance in the non-debug case doesn't change
(we still just check one condition up front), and I think the impact is
small enough in the debug context case to warrant including all of it.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>