drm/xe/stolen: Exclude reserved lmem portion

The address set by firmware in GEN12_DSMBASE in driver initialization
doesn't mean "anything above that and until end of lmem is part of DSM".
In fact, there may be a few KB that is not part of DSM on the end of
lmem. How large is that space is platform-dependent, but since it's
always less than the DSM granularity, it can be simplified by simply
aligning the size down.

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