drm/i915: Defer removing fence register tracking to rpm wakeup

Currently, we may simultaneously release the fence register from both
fence_update() and i915_gem_restore_fences(). This is dangerous, so
defer the bookkeeping entirely to i915_gem_restore_fences() when the
device is asleep.

Reported-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190208153708.20023-1-chris@chris-wilson.co.uk