drm/i915: setup io-mapping for LMEM

Create an io-mapping to describe the CPU aperture for lmem.

Signed-off-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191025153728.23689-2-chris@chris-wilson.co.uk