OMAPDSS: APPLY: clean up isr_handler

Add a helper function mgr_clear_shadow_dirty(), which clears the
shadow_dirty flags for the given manager and for the overlays on that
manager.

This lets us simplify the code in the dss_apply_irq_handler().

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