drm/sun4i: Fix an ulong overflow in the dotclock driver

The calculated ideal rate can easily overflow an unsigned long, thus
making the best div selection buggy as soon as no ideal match is found
before the overflow occurs.

Fixes: 4731a72d ("drm/sun4i: request exact rates to our parents")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181018100250.12565-1-boris.brezillon@bootlin.com