drm/i915: Create stolen memory region from local memory

Add "REGION_STOLEN" device info to dg1, create stolen memory
region from upper portion of local device memory, starting
from DSMBASE.

v2:
    - s/drm_info/drm_dbg; userspace likely doesn't care about stolen.
    - mem->type is only setup after the region probe, so setting the name
      as stolen-local or stolen-system based on this value won't work. Split
      system vs local stolen setup to fix this.
    - kill all the region->devmem/is_devmem stuff. We already differentiate
      the different types of stolen so such things shouldn't be needed
      anymore.
v3:
    - split stolen lmem vs smem ops(Tvrtko)
    - add shortcut for stolen region in i915(Tvrtko)
    - sanity check dsm base vs bar size(Xinyun)
v4(Tvrtko):
    - more cleanup
    - add some TODOs

Signed-off-by: CQ Tang <cq.tang@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Xinyun Liu <xinyun.liu@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210421104658.304142-1-matthew.auld@intel.com