OMAPDSS: APPLY: fix clearing shadow dirty flag with manual update

Currently the shadow-dirty flags for manual update displays is cleared
in the apply_irq_handler when an update has finished. This is not
correct, as the shadow registers are taken into use (i.e. after that
they are not dirty) when the update is started.

Move the mgr_clear_shadow_dirty() call from apply_irq_handler to
dss_mgr_start_update() to fix this.

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