drm/omap: remove all EXPORT_SYMBOLs from dispc.c

omapdrm now uses dispc_ops instead of direct function calls so we can
remove all EXPORT_SYMBOLs from dispc. Most of the functions can also be
made static, but a few are used outside dispc.c.

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