drm/omap: add omapdss-base.ko

drm/omap: add omapdss-base.ko

We are working towards enabling omapdss6, which will consists of a new
dss, dispc and dpi drivers. omapdss6 will be a new module. The panel,
encoder and omapdrm will need to use either the current omapdss driver
or the new omapdss6 driver, depending on the platform.

This will be implemented with a common base module and function
pointers.

This patch adds a skeleton omapdss-base.ko module, to which we'll be
moving common dss functionality like registration of the panels.

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