omapdss: remove CONFIG_OMAP2_DSS_VENC from omapdss.h

We are about to make a private copy of omapdss for omapfb and for
omapdrm. The omapdss.h file will still be shared for the time being. The
copy will need to change the config symbols, and we happen to have one
use of these config symbols in omapdss.h which needs to be removed.

Luckily we can just delete the use of CONFIG_OMAP2_DSS_VENC from
omapdss, as it's used only to hide extern declarations, and there's no
harm to have those declarations even if CONFIG_OMAP2_DSS_VENC is not
set.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Acked-by: Rob Clark <robdclark@gmail.com>