OMAP: DSS2: Move DPI & SDI init into DSS plat driver

DPI and SDI are different from the other interfaces as they are not
hwmods and there is not platform driver for them. They could be said to
be a part of DSS or DISPC modules, although it's not a clear definition.

This patch moves DPI and SDI initialization into DSS platform driver,
making the code more consistent: omap_dss_probe() only initializes
platform drivers now.

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