modesetting: Fix VRR window property handling.

A misplaced error check can cause this failure scenario, and does
so reliably as tested on Ubuntu 21.04 with KDE Plasma 5 desktop
within the first few seconds of login session startup, rendering
VRR under modesetting-ddx unusable:

1. Some X11 client application changes some window property.

2. ms_change_property() is called as part of the property change
   handling call chain (client->requestVector[X_ChangeProperty]).
   It removes itself temporarily from the call chain - or so it
   thinks, hooking up saved_change_property instead.

3. ret = saved_change_property(client) is called and fails
   temporarily for some non-critical reason.

4. The misplaced error check returns early (error abort), without
   first restoring ms_change_property() as initial X_ChangeProperty
   handler in the call chain again.

-> Now ms_change_property() has removed itself permanently from the
   property handler call chain for the remainder of the X session
   and VRR property changes on windows are no longer handled, ie.
   VRR no longer gets enabled/disabled in response to window VRR
   property changes.

Place the error check at the proper place, just as it is correctly
done by amdgpu-ddx, and in modesetting-ddx ms_delete_property()
function.

Verified to fix VRR handling with an AMD gpu under KDE desktop
session.

Please consider merging before branching the server 1.21 branch.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>