drm/xe/ufence: Warn if mmget_not_zero() fails

This shouldn't happen but seen this while debugging ufence timeout
issue time to time so log it to isolate this particular case.

v2: s/XE_WARN_ON/drm_dbg(Maarten)

Link: #1630


Cc: Francois Dugast <francois.dugast@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241016082304.66009-3-nirmoy.das@intel.com


Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>