media: ov2680: Add runtime-pm support

Remove the obsolete s_power() callback and instead use runtime-pm +
autosuspend, powering-on the sensor on s_stream(1) and releasing
the runtime-pm reference on s_stream(0).

This also removes the need for ov2680_mode_restore() instead
ov2680_stream_enable() now takes care of all sensor initalization
after power-on.

This is a preparation patch for adding ACPI support.

Note this also removes putting the clock lane into LP-11 state from
ov2680_power_on() since now streaming will start immediately after
powering on the sensor there is no need to put the clock lane
in a low power state.

Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>