media: sun6i-csi: Split stream sequences and irq code in capture

Create minimal helpers that split the enable/disable flow, which will
make it easier to move control over to the bridge later on.

Generally speaking the goal is to move register configuration to
the capture code and later split it with the bridge code.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>