drm/xe: Implement stolen memory.

This adds support for stolen memory, with the same allocator as
vram_mgr. This allows us to skip a whole lot of copy-paste,
by re-using parts of xe_ttm_vram_mgr.

The stolen memory may be bound using VM_BIND, so it performs like any
other memory region.

We should be able to map a stolen BO directly using the physical memory
location instead of through GGTT even on old platforms, but I don't know
what the effects are on coherency.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>