anv,iris: Use I915_MMAP_OFFSET_FIXED for LMEM platforms
@kwg, please take a look. I'm not sure I really like the way I plumbed it through iris. There's a part of me that thinks that, instead of adding a new IRIS_MMAP_FIXED, we should instead add asserts that the IRIS_MMAP_WB/WC matches the i915 uAPI expectations and otherwise ignore the iris mmap mode.