drm/fb-helper: Fix typo in parameter description

Fix typo in parameter description.

Fixes: 4be9bd10

 ("drm/fb_helper: Allow leaking fbdev smem_start")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1543905135-35293-1-git-send-email-weiyongjun1@huawei.com