OMAPDSS: APPLY: make ovl_enable/disable synchronous

ovl->enable/disable are meant to be synchronous so that they can handle
the configuration of fifo sizes. The current kernel doesn't configure
fifo sizes yet, and so the code doesn't need to block to function (from
omapdss driver's perspective).

However, for the users of omapdss a non-blocking ovl->disable is
confusing, because they don't know when the memory area is not used
any more.

Furthermore, when the fifo size configuration is added in the next merge
window, the change from non-blocking to blocking could cause side
effects to the users of omapdss. So by making the functions block
already will keep them behaving in the same manner.

And, while not the main purpose of this patch, this will also remove the
compile warning:

drivers/video/omap2/dss/apply.c:350: warning:
'wait_pending_extra_info_updates' defined but not used

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>