... and I immediately regret that I've killed the return value
for igt_debugfs_init, since we have callers which need to work
without the forcewake stuff, e.g. the reg dumper needs to work
without i915 loaded.

Put this new helper to good use in the mmio code and the pm_pc8
testcase.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>