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_ctx_exec.c

@@ -46,7 +46,7 @@
 #include "ioctl_wrappers.h"
 #include "drmtest.h"
 #include "igt_aux.h"
-#include "igt_debugfs.h"
+#include "igt_gt.h"
 
 IGT_TEST_DESCRIPTION("Test basic context switch functionality.");