drm/i915: Factor out i915_ggtt_suspend_vm/i915_ggtt_resume_vm()

Factor out functions that are needed by the next patch to suspend/resume
the memory mappings for DPT FBs.

No functional change, except reordering during suspend the
ggtt->invalidate(ggtt) call wrt. atomic_set(&ggtt->vm.open, open) and
mutex_unlock(&ggtt->vm.mutex). This shouldn't matter due to the i915
suspend sequence being single threaded.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211101183551.3580546-1-imre.deak@intel.com