drm/xe: Rename xe_engine.[ch] to xe_exec_queue.[ch]

This is a preparation commit for a larger renaming of engine to exec queue.

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>