drm/i915: Remove temporary RPM wakeref assert disables

Now that the last couple of hacks have been removed from the runtime
powermanagement users, we can fully enable the asserts by preventing the
temptation to disable them when our code is buggy.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1468055535-19740-11-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>