lcd: add clear and draw bitmap declaration

The functions for clearing and drawing bitmaps on the screen were not
exposed publicly and are made public in this patch in preparation for
implementing the display interface of api_public.h.

Signed-off-by: Che-Liang Chiou <clchiou@chromium.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Anatolij Gustschin <agust@denx.de>