drm/omap, omapfb: move exported dispc function declarations to omapdrm/omapfb

omapdrm and omapfb still share the same include/video/omapdss.h. We need
to change that so that we can proceed with omapdrm work.

However, it's not trivial to make separate omapfb and omapdrm versions
of omapdss.h, as that file is also included in other places like arch
code, audio code and omap_vout code. So we'll do it piece by piece.

This patch makes private versions of all the dispc function declarations
that are in omapdss.h. For omapdrm we create a new file,
drivers/gpu/drm/omapdrm/dss/omapdss.h, which will contain headers meant
to be visible outside omapdss.

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