drm/xe: Rename engine to exec_queue

Engine was inappropriately used to refer to execution queues and it
also created some confusion with hardware engines. Where it applies
the exec_queue variable name is changed to q and comments are also
updated.

Link: drm/xe/kernel#162


Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>