OMAP: DSS2: make 50ms bug-fix sleep optional

For some unknown reason we may get SYNC_LOST errors from the display
subsystem at initialization time if we don't sleep before resetting the
DSS. See the source (dss.c) for more comments.

However, 50ms is quite long time to sleep, and with some configurations
the SYNC_LOST may never happen, so this patch creates a Kconfig option
to disable the sleep.

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