drm/xe/pf: Clear all LMTT pages on alloc

Our LMEM buffer objects are not cleared by default on alloc
and during VF provisioning we only setup LMTT PTEs for the
actually provisioned LMEM range. But beyond that valid range
we might leave some stale data that could either point to some
other VFs allocations or even to the PF pages.

Explicitly clear all new LMTT page to avoid the risk that a
malicious VF would try to exploit that gap.

While around add asserts to catch any undesired PTE overwrites
and low-level debug traces to track LMTT PT life-cycle.

Fixes: b1d20405 ("drm/xe/pf: Introduce Local Memory Translation Table")
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Lukasz Laguna <lukasz.laguna@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://lore.kernel.org/r/20250701220052.1612-1-michal.wajdeczko@intel.com