OMAPDSS: fix DPI & DSI init order

DPI may use DSI PLL, so it depends on DSI. However, currently DPI driver
is added first, which causes DPI initialization to fail when it tries to
get the DSI PLL.

This patch changes the init order to fix this.

A better solution would be to separate DSI PLL and DSI drivers. They
have dependencies, though, but we could still have DSI PLL as an
independent entity that we could initialize before any of the output
drivers.

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