OMAPDSS: DPI: verify if DSI PLL is operational

The SoCs that have DSI module should have a working DSI PLL. However,
some rare boards have not connected the powers to the DSI PLL.

This patch adds a function that tries to power up the DSI PLL, and
reports if that doesn't succeed. DPI uses this function to fall back to
PRCM clocks if DSI PLL doesn't work.

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