drm/i915: Kill the fake lmem support

This was useful for early development of lmem, but it's not used
anymore, so remove it.

v2: Remove unneeded fields from struct intel_memory_region

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220217175634.4128754-1-lucas.demarchi@intel.com