[Haswell] HD Graphics 4600: Provide a flag / switch to disable CVE-2019-14615 mitigation.

As has been widely reported the renderclear patch for gen7 + 7.5 has taken a ~40% bite out of iGPU performance. On the kernel we can of course run mitigations=off at our choosing but no such option exists for the GPU drivers. The patch has also affected other generations but only minimally.

There was supposed to be a updated version which didn't strain performance so much but it likely fell by the wayside. Can we at least have a switch much like the other module switches to opt out of this security fix?

On desktop systems we can easily upgrade or install a different GPU but this is not the case for notebooks. Some popular ones use this chip, including the last thinkpad with a replaceable CPU (t440P). Its the last machine you can use with coreboot as well. Not having to compile every single kernel release or missing out on other more important security fixes has not been a great option.

Offending mitigation is here: https://lists.freedesktop.org/archives/dri-devel/2020-January/252142.html