OMAP: DSS2: DSI: sync when disabling a display

When the panel driver calls omapdss_dsi_display_disable() it is possible
that there are still some unsent packets in the TX fifo.

Add dsi_sync_vc() calls in the beginning of
omapdss_dsi_display_disable() to make sure the TX fifos are empty.

This allows us to remove the msleep(10) hack from panel-taal.c

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