I am not familiar with Arch Linux, but Gentoo Linux's documentation briefly describes this issue:
The unofficial wiki is a bit more helpful in describing it:
Gentoo Linux users typically compile their own kernels, so they do not use those parameters I posted, but as long as you have CONFIG_MTRR_SANITIZER in your kernel .config, you can use those parameters to enable it.
By the way, MTRR has nothing to do with your graphics card. It is an x86 feature. All x86 processors support it.