drm/xe: Fix dequeue of access counter work item

The access counters worker function is fixed to advance the head pointer
when dequeuing from the acc_queue.  This now matches the similar logic in
get_pagefault().

Signed-off-by: Bruce Chang <yu.bruce.chang@intel.com>
Signed-off-by: Brian Welty <brian.welty@intel.com>
Reviewed-by: Stuart Summers <stuart.summers@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>