[media] pwc: Read new preset values when changing awb control to a preset

So that events get generated for the new red + blue bal values when switching
to a preset. This allows apps to solely rely on events instead of needing
to do a query + g_ctrl on all controls when a control with the update flag
set is gets set or gets an event.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>