drm/i915/mtl: Fix MTL stolen memory GGTT mapping

The PTEs expect the offset from the base of the fake LMEM region (i.e.
the base of stolen) and not from the base of the DSM. Quoting the specs:
"Driver will set the Device Memory bit = 1 in the PTE when pointing to a
page in DSM and program the PTE with offset from LMEM_BAR. Device Memory
Offset from LMEM_BAR is same as offset from BGSM."

DSM starts 8MBs from BGSM, so we set dsm_base = 8MB.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Aravind Iddamsetty <aravind.iddamsetty@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Cc: Fei Yang <fei.yang@intel.com>
Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230328012430.2524330-1-daniele.ceraolospurio@intel.com