drm: xlnx: fix spelling mistake "failes" -> "failed"

There is a spelling mistake in a dev_dbg messages. Fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.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/20200724111258.14762-1-colin.king@canonical.com