lib/igt_gt: Document and consolidate

Also move forcewake and stop_rings code from igt_debugfs to igt_gt
since it fits better. And move the hang injection fork helpers from
igt_aux to igt_gt, too.

Also push the intel_gen call into igt_hang_ring while at it.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

tests/gem_evict_alignment.c

@@ -48,6 +48,7 @@
 #include "drmtest.h"
 #include "intel_chipset.h"
 #include "igt_aux.h"
+#include "igt_gt.h"
 
 IGT_TEST_DESCRIPTION("Run a couple of big batches to force the unbind on"
 		     " misalignment code.");