drm/i915/selftests: Fix comment typo

Fix the double `wait' typo in comment.

Signed-off-by: Jason Wang <wangborong@cdjrlc.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220716040520.31676-1-wangborong@cdjrlc.com