drm/omap: add dispc_ops

We want to change the dispc API from plain functions to a struct with
functions pointers, so that omapdrm can call either omapdss or omapdss6
depending on the platform.

This patch adds 'struct dispc_ops' and adds functions to omapdss-base
to set and get the ops.

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