media: rockchip: rga: set dma mask to 32 bits

The RGA DMA descriptor list contains only 32-bit addresses. Set the
dma_mask to only allocate memory that is addressable by the descriptors.

This prevents errors when preparing vb2 buffers that were allocated by
the RGA.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>