media: staging: rkisp1: cap: move code that manages the buffers to rkisp1_set_next_buf

The function 'rkisp1_set_next_buf' configures the registers
according to 'cap->buf.next'. It is called after updating
'cap->buf.next' and 'cap->buf.curr'. This patch moves the
code that updates those fields to rkisp1_set_next_buf.
This is a preparation for later patch that change a call to
'rkisp1_handle_buffer' with a call to 'rkisp1_set_next_buf'.

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Acked-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>