drm/xe/svm: Add stats for SVM page faults

drm/xe/svm: Add stats for SVM page faults

Add a new entry in stats to for svm page faults. If CONFIG_DEBUG_FS is
enabled, the count can be viewed with per GT stat debugfs file.

This is similar to what is already in place for vma page faults.

Example output:

    cat /sys/kernel/debug/dri/0/gt0/stats
    svm_pagefault_count: 6
    tlb_inval_count: 78
    vma_pagefault_count: 0
    vma_pagefault_kb: 0

v2: Fix build with CONFIG_DRM_GPUSVM disabled

v3: Update argument in kernel doc

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250312092749.164232-1-francois.dugast@intel.com
Signed-off-by: Francois Dugast <francois.dugast@intel.com>