drm/i915: move region_lmem under gt

Device local-memory should be thought of as part the GT, which means it
should also sit under gt/.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: 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/20210112164300.356524-1-matthew.auld@intel.com