libcamera: device_enumerator: fix udev media graph loading dependency

When a MediaDevice is enumerated and populated by the
DeviceEnumeratorUdev, there is a possibility that the member device
nodes of the media graph would not be ready (either not created, or
without proper permissions set by udev yet). The MediaDevice is still
passed up to the pipeline handler, where an attempt to access the device
nodes will fail in EPERM. This whole issue is especially likely to
happen when libcamera is run at system init time.

To fix this, we first split DeviceEnumerator::addDevice() into three
methods:
- createDevice() to simply create the MediaDevice
- populateMediaDevice() to populate the MediaDevice
- addDevice() to pass the MediaDevice up to the pipeline handler

DeviceEnumeratorSysfs calls these methods in succession, similar to what
it did before when they were all together as addDevice().

DeviceEnumeratorUdev additionally keeps a map of MediaDevices to a list
of pending device nodes (plus some other auxillary maps), and a simple
list of orphan device nodes. If a v4l device node is ready and there
does not exist any MediaDevice node for it, then it goes to the orphan
list, otherwise it is initialized and removed from the pending list of
the corresponding MediaDevice in the dependency map. When a MediaDevice
is populated via DeviceEnumeratorUdev::populateMediaDevice(), it first
checks the orphan list to see if the device nodes it needs are there,
otherwise it tries to initialize the device nodes and if it fails, then
it adds the device nodes it wants to its list in the dependency map.

This allows MediaDevice instances to be created and initialized properly
with udev when v4l device nodes in the media graph may not be ready when
the MediaDevice is populated.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

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