omapfb: Create new header file for omapfb DSS implementation

Copy the content of video/omapdss.h to a new (video/omapfb_dss.h) header
file and convert the omapfb drivers to use this new file.

The new header file is needed to complete the separation of omapdrm and
omapfb implementation of DSS.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>