OMAP: DSS2: DSI: use BTA to end the frame transfer

Previously a work was started on FRAMEDONE interrupt, and this work
either sent a BTA synchronously or looped until TE_SIZE was zero, to
wait for the end of the transfer.

This patch changes a BTA to be sent asynchronously from FRAMEDONE
interrupt, and when a BTA interrupt is received, the transfer is
finished. This way we do the whole process asynchronously, and also
inside interrupt context.

This will give us much better latency to handle the end of the frame
than with the previous work based solution.

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