drm/xe: Remove references to CONFIG_DRM_XE_DEVMEM_MIRROR

The prefetch code was referencing CONFIG_DRM_XE_DEVMEM_MIRROR, which has
been replaced by CONFIG_DRM_XE_PAGEMAP. As a result, prefetches were
limited to SRAM. Update the code to use CONFIG_DRM_XE_PAGEMAP instead of
the deprecated option.

Fixes: f86ad0ed ("drm/gpusvm, drm/pagemap: Move migration functionality to drm_pagemap")
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20250710205413.1105595-1-matthew.brost@intel.com