drm/xe: Add more GT stats around pagefault mode switch flows

drm/xe: Add more GT stats around pagefault mode switch flows

Add GT stats to measure the time spent switching between pagefault mode
and dma-fence mode. Also add a GT stat to indicate when pagefault
suspend is skipped because the system is idle. These metrics will help
profile pagefault workloads while 3D and display are enabled.

v2:
 - Use GT stats helper functions (Francois)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Link: https://patch.msgid.link/20251212182847.1683222-8-matthew.brost@intel.com