drm/xe/configfs: Allow to enable PSMI

Now that additional WAs are in place and it's possible to allocate
buffers through debugfs, add the configfs attribute to turn PSMI on.

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Riana Tauro <riana.tauro@intel.com>
Link: https://lore.kernel.org/r/20250821-psmi-v5-7-34ab7550d3d8@intel.com


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