drm: xlnx: Fix typo in parameter description

Fix typo in parameter description.

Fixes: d76271d2 ("drm: xlnx: DRM/KMS driver for Xilinx ZynqMP DisplayPort Subsystem")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200725063429.172139-1-weiyongjun1@huawei.com