OMAPDSS: DPI: always use DSI PLL if available

We currently get the decision whether to use PRCM or DSI PLL clock for
DPI from the board file. This is not a good way to handle it, and it
won't work with device tree.

This patch changes DPI to always use DSI PLL if it's available.

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