OMAPDSS: DSI: improve DSI clock calcs for DISPC

Commit ee144e64 added
dsi_pll_calc_ddrfreq() which calculates PLL dividers based on given DSI
bus clock speed. The function works ok, but it can be improved for the
DISPC clock calc.

The current version calculates the clock going from the PLL to the DISPC
simply by setting the clock as close to DISPC maximum as possible, and
the pixel clock is calculated based on that.

This patch changes the function to calculate DISPC clock more
dynamically, iterating through different DISPC clocks and pixel clock
values, and thus we'll get more suitable pixel clocks.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>