drm/i915/gt: fix spelling mistake "undeflow" -> "underflow"

There is a spelling mistake in comment, fix it.

Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200401022506.52965-1-chenzhou10@huawei.com