drm/xe: Use q->xef for accessing xe file

No need to traverse through the vm object as each exec queue maintains a
reference to xe_file. Also improve/simplify the comment on why xef is
checked.

Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241218053122.2730195-2-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>