x11-drivers/xf86-video-ati: Fix missing hw cursor

Add an upstream patch (submitted by myself and danfe) to fix the issue where
the hw cursor sometimes does not work when using x11-drivers/xf86-video-ati.
Big thanks to danfe@ for debugging and testing and figuring out what's going
on.

For details on the change, see upstream issue:
https://gitlab.freedesktop.org/xorg/driver/xf86-video-ati/-/issues/190

PR: 237642
Reported by: jwb
MFH: 2020Q2