gpu: ipu-v3: allocate ipuv3_channels as needed

Most of the 64 IPUv3 DMA channels are never used, some of them (channels
16, 30, 32, 34-39, and 53-63) are even marked as reserved.
Allocate the channel control structure only when a channel is actually
requested, replace the fixed size array with a list, and remove the
unused enabled and busy fields from the ipuv3_channel structure.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>