OMAP: DSS2: Check ctx loss count only when starting the first clock

When OMAP PM layer is no-op/debug, the PM layer will increment context
loss count with every call. This resulted DSS2 to restore context
whenever a clock was enabled.

This commit checks the context loss count only when the context actually
could have been lost, ie. when enabling a clock when no clocks had been
previously enabled.

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