drm/xe: Make LRC W/A scratch buffer usage consistent

drm/xe: Make LRC W/A scratch buffer usage consistent

The LRC W/A currently checks for LRC being iomem in some places, while
in others it checks if the scratch buffer is non-NULL. This
inconsistency causes issues with the VF post-migration recovery code,
which blindly passes in a scratch buffer.

This patch standardizes the check by consistently verifying whether the
LRC is iomem to determine if the scratch buffer should be used.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Tomasz Lis <tomasz.lis@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20251008214532.3442967-8-matthew.brost@intel.com