media: subdev: Improve v4l2_subdev_enable/disable_streams_fallback

v4l2_subdev_enable/disable_streams_fallback() supports falling back to
.s_stream() for subdevs with a single source pad. It also tracks the
enabled streams for that one pad in the sd->enabled_streams field.

Tracking the enabled streams with sd->enabled_streams does not make
sense, as with .s_stream() there can only be a single stream per pad.
Thus, as the v4l2_subdev_enable/disable_streams_fallback() only supports
a single source pad, all we really need is a boolean which tells whether
streaming has been enabled on this pad or not.

However, as we only need a true/false state for a pad (instead of
tracking which streams have been enabled for a pad), we can easily
extend the fallback mechanism to support multiple source pads as we only
need to keep track of which pads have been enabled.

Change the sd->enabled_streams field to sd->enabled_pads, which is a
64-bit bitmask tracking the enabled source pads. With this change we can
remove the restriction that
v4l2_subdev_enable/disable_streams_fallback() only supports a single
source pad.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Umang Jain <umang.jain@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>