drm/bridge/sii8620: fix potential buffer overflow

Buffer overflow error should not occur, as mode_fixup() callback
filters pixel clock value and it should never exceed 600000. However,
current implementation is not obviously safe and relies on
implementation of mode_fixup().

Make 'i' variable never reach unsafe value in order to avoid buffer
overflow error.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: bf1722ca

 ("drm/bridge/sii8620: rewrite hdmi start sequence")
Signed-off-by: Maciej Purski <m.purski@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1511341718-6974-1-git-send-email-m.purski@samsung.com