Allow configuring kernel with iommu=igfx_off by default, with cmdline optional override to igfx_on

[Dimitri John Ledkov]

[Impact]

 * it is desired to have intel_iommu=on by default
 * However this is causing many gpu bugs which are resolvable with igfx_off parameter
 * there is no Kconfig that can have intel_iommu=igfx_off behaviour
 * introduce intel_iommu=igfx_on kernel command line
 * introduce Kconfig that allows to have iommu default igfx_off
 * turn both of them on

[Test Plan]

 * Test that iommu is on by default
 * Test that igfx_off is off by default
 * Test that one can turn on GPU iommu with igfx_on

[Where problems could occur]

 * Lots of GPU bugs are discovered each time we try to turn IOMMU by default, hopefully with igfx_off we might have more success in turning iommu on and keeping it on. See https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1965882 https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1971146 https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1971699
https://twitter.com/hughsient/status/1564335142243602432

[Other Info]

 * Rock & a hard place => our users expect working displays in default kernel config which IOMMU seems to break a lot.

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is missing log files that will aid in diagnosing the problem. While running an Ubuntu kernel (not a mainline or third-party kernel) please enter the following command in a terminal window:

apport-collect 1988831

and then change the status of the bug to 'Confirmed'.

If, due to the nature of the issue you have encountered, you are unable to run this command, please add a comment stating that fact and change the bug status to 'Confirmed'.

This change has been made by an automated script, maintained by the Ubuntu Kernel Team.