Make use of property request wrappers for VRR property

Instead of scanning for PropertyNotify events. Reasons:

* Works even if no client listens to PropertyNotify events for the
  window.
* No overhead on delivery of unrelated events, and no overhead at all
  if Option "VariableRefresh" is disabled.

v2:
* Use shorter variable name amdgpu_vrr_atom.
* Call MakeAtom regardless of info->instance_id, for robustness vs VRR
  being enabled in some but not all AMDGPU screens.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>