[media] pxa_camera: trivial move of dma irq functions

This moves the dma irq handling functions up in the source file, so that
they are available before DMA preparation functions. It prepares the
conversion to DMA engine, where the descriptors are populated with these
functions as callbacks.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>