media: qcom: camss: Power pipeline only when streaming

The libcamera plugin for Pipewire may keep an open file descriptor to
the video device, even while streaming. This simplifies its operation,
as it only needs to keep track of a number instead of a file path. When
the video device is open but not streaming, the pipeline can be powered
off. Move the pipeline power management to the prepare_streaming and
unprepare_streaming functions.

Signed-off-by: Richard Acayan <mailingradian@gmail.com>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Tested-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Vladimir Zapolskiy <vladimir.zapolskiy@linaro.org>
Signed-off-by: Bryan O'Donoghue <bod@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>