davinci: timers: don't enable timer until clocksource is initialized

On da830, when the same timer is used for clocksource and clockevent,
the timer can be started before the clockevent is
registered/initialzed.  This creates a window where a timer
interrupt might fire before the clockevent handler has been
setup and causes a crash.

This patch moves the actual enable/start of the timer after
the clockevent has ben registered.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>