drm/i915: factor out i915_drm_suspend_late

This is needed by an upcoming patch fixing the switcheroo/legacy suspend
paths.

No functional change.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>