drm/mediatek: fix a timeout loop

This code causes a static checker warning because it treats "i == 0" as
a timeout but, because it's a post-op, the loop actually ends with "i"
set to -1.  Philipp Zabel points out that it would be cleaner to use
readl_poll_timeout() instead.

Fixes: 21898816 ("drm/mediatek: add dsi transfer function")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>