drm/i915/gem: Support discontiguous lmem object maps

Create a vmap for discontinguous lmem objects to support
i915_gem_object_pin_map().

v2: Offset io address by region.start for fake-lmem

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200102204215.1519103-1-chris@chris-wilson.co.uk