drm/i915: give stolen system memory its own class

In some future patches we will need to also support a stolen region
carved from device local memory, on platforms like DG1. To handle this
we can simply describe each in terms of its own memory class.

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/20210205102026.806699-2-matthew.auld@intel.com


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>