drm/i915: support creating LMEM objects

We currently define LMEM, or local memory, as just another memory
region, like system memory or stolen, which we can expose to userspace
and can be mapped to the CPU via some BAR.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Abdiel Janulgue <abdiel.janulgue@linux.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/20191025153728.23689-1-chris@chris-wilson.co.uk