coreboot/dell9020*_12mb: Disable IOMMU by default

Needed to make graphics cards work. Turn it on if you're using
only the Intel GPU.

With IOMMU *enabled*, graphics cards do not work reliably at all.
The cause still needs to be investigated, but the symptoms are
graphical corruption on the screen, and Xorg usually crashes.
In some cases (on some cards), TTYs can still be used; the payload
can still be used reliably, on a graphics card, but Xorg fails to
work properly.

This could be a bug in Linux drivers, instead of anything that
coreboot does (not yet tested in factory BIOS).

Leaving it off by default will ensure reliable operation on all
setups, whether an iGPU or dGPU is used.

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/coreboot/default/patches/0037-dell-optiplex_9020-Disable-IOMMU-by-default.patch

@@ -0,0 +1,29 @@
+From 61041d49b94236400e836b8ea518d3a064b95c4e Mon Sep 17 00:00:00 2001
+From: Leah Rowe <info@minifree.org>
+Date: Sat, 2 Mar 2024 23:00:09 +0000
+Subject: [PATCH 2/2] dell/optiplex_9020: Disable IOMMU by default
+
+Needed to make graphics cards work. Turning it on is
+recommended if only using iGPU, otherwise leave it off
+by default. The IOMMU is extremely buggy when a graphics
+card is used. Leaving it off by default will ensure that
+the default ROM images in Libreboot will work on any setup.
+
+Signed-off-by: Leah Rowe <info@minifree.org>
+---
+ src/mainboard/dell/optiplex_9020/cmos.default | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/src/mainboard/dell/optiplex_9020/cmos.default b/src/mainboard/dell/optiplex_9020/cmos.default
+index 8253570f19..7bccc80e51 100644
+--- a/src/mainboard/dell/optiplex_9020/cmos.default
++++ b/src/mainboard/dell/optiplex_9020/cmos.default
+@@ -2,4 +2,4 @@ boot_option=Fallback
+ debug_level=Debug
+ nmi=Disable
+ power_on_after_fail=Disable
+-iommu=Enable
++iommu=Disable
+-- 
+2.39.2
+