lib: (somewhat) structured logging support

lib: (somewhat) structured logging support

Apparently there's a bit a need for more verbose output in testcases,
mostly for debugging purposes. At least gem_reset_stats and pm_rps
have a verbose mode.

On top of that we're currently not taking advantage of piglit's "warn"
state all that much. But I think it might be useful for testcases which
are notorious for some kinds of spurious failures, like e.g. the really
nasty timing checks in kms_flip. If we demote some of them to just
warnings we could run the overall tests more often.

Hence this patchs adds a new igt_log function with the three levels DEBUG,
INFO and WARN. Plus a bunch of convenience helpers to keep the test
code tidy.

The level can be set through an enviroment vairable IGT_LOG_LEVEL with
info being the default. Also tests can look at the selected log level in
case they want to run costly debug functions only when needed.

Comments highly welcome, I plan to roll this out over tests which can
use it (not all, imo that's too much churn) once we've settled on the
interfaces/semantics.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>