drm/vc4: Correct DSI divider calculations

The divider calculations tried to find the divider
just faster than the clock requested. However if
it required a divider of 7 then the for loop
aborted without handling the "error" case, and could
end up with a clock lower than requested.

Correct the loop so that we always have a clock greater
than requested.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>