drm/i915: Remove leftover gem.pm_notifier member

Since we removed the pm hookup from the GT, the hook in
drm_i915_private.gem is unused. Remove it.

References: 18f3b272 ("drm/i915: Remove pm park/unpark notifications")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191112113434.31088-1-chris@chris-wilson.co.uk