drm/xe: Invert page fault queue head / tail

Convention for queues in Linux is the producer moves the head and
consumer moves the tail. Fix the page fault queue to conform to this
convention.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>