OMAPDSS: APPLY: Remove unnecessary variable in dss_apply_irq_handler

The bool was_updating is never really used for anything. It is set to the
current value of mp->updating, but not used anywhere. Remove this variable.

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