need_isr() should return true whenever there's work that needs to be
handled in the VSYNC/FRAMEDONE irq handler. However, need_isr() is
missing some cases where we need the irq handler.

This patch adds checks for shadow dirty flags, so that the irq handler
is started/kept running while shadow dirty flags are set.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>