When applications modify preview engine parameters, the new values are
applied to the hardware by the preview engine interrupt handler during
vertical blanking. If the parameters are being changed when the
interrupt handler is called, it just delays applying the parameters
until the next frame.

If an application modifies the parameters for every frame, and the
preview engine interrupt is triggerred synchronously, the parameters are
never applied to the hardware.

Fix this by storing new parameters in a shadow copy, and switch the
active parameters with the shadow values atomically.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>