x11-servers/xorg-server: Restore FIXDRM as an off-by-default knob

This is a workaround for a problem with certain systems [1] after
x11-servers/xorg-server was upgraded to 1.20.7. Other workarounds are
described in PR 244306.

[1]
These systems have been reported to have problems:
Sandy Bridge
Ivy Bridge
Broadwell
Kaby Lake
Whiskey Lake

PR: 244306
Submitted by: wulf
Reported by: philippe.michel7@free.fr
Approved by: x11 (zeising)
Differential Revision: https://reviews.freebsd.org/D23834