To return to default, you must edit "Preferences.xml", and remove the
entry at the end of the XML file.
Notably, the iHD driver has been patched on most Synology units, so you might not see a difference... and if you are streaming/transcoding 4k, the patched iHD driver will outperform the i965 driver (the video does not address this detail).
I found the long form of the NAS Compares instructions and there is a serious typo that will cause a lot of grief if you don't have a backup of the Plex Media Server library folder.
In step 12 the text to insert is given as: VaapiDriver=”i965″ />
Note both double quotes, they are not compliant with the file's text format and when saved will result in most of the Preferences.xml being lost. The normal double quote character should be used "". Hyper Backup is your friend here.
I was having pixelation when transcoding 1080p MKV on the DS1520+ so took the plunge on this as a test. Using a text editor via SFTP, though I see that Text Editor in DSM does provide an alert (in subsequent investigation): if you get an alert when saving please stop and check the double quotes. Oh and you can't just rollback Preferences.xml it took a restore of the full folder.
For me I can confirm that this 'fix', when properly done, does correct pixelation also on MKV/H.264. Plus I found playback on DS video to be problematic without the 'fix'.*