drm/xe: Carve out top of DSM as reserved

Top of DSM contains the WOPCM where kernel driver shouldn't access as
it contains data from other HW agents. Carve it out from the stolen
memory. On a MTL system, the output now matches the expected values:

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20230726160708.3967790-10-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>