OMAPDSS: fix rounding when calculating fclk rate

"clk: divider: fix rate calculation for fractional rates" patch (and
similar for TI specific divider) fixes the clk-divider's rounding. This
patch updates the DSS driver to round the rates accordingly.

This fixes the DSS's warnings about clock rate mismatch, and also fixes
the wrong fclk rate being set.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: Christoph Fritz <chf.fritz@googlemail.com>
Tested-by: Marek Belisko <marek@goldelico.com>