video: omap: set LCD clock rate through DM API

The patch configures the display DPLL using the functions provided by
the driver model API for the clock. The device tree contains everything
needed to get the DPLL clock. The round rate function developed for
calculating the DPLL multiplier and divisor and the platform routines
for accessing the DPLL registers are removed from the LCD driver code
because they are implemented inside the DPLL clock driver.

Signed-off-by: Dario Binacchi <dariobin@libero.it>