ACPI: video: Rework checking for duplicate video bus devices

The current way of checking for duplicate video bus devices in
acpi_video_bus_probe() is based on walking the ACPI namespace which is
not necessary after recent driver conversions.  It is also susceptible
to race conditions (for example, if two video bus devices are probed at
the same time) and ordering issues.

Instead of doing it the old way, inspect the children of the parent of
the device being probed, excluding the latter and the children that are
not auxiliary devices.  For each of the remaining children, check if any
of the entries in the video_bus_head list is equal to its driver data
which can only happen if the given child has been processed by
acpi_video_bus_probe() successfully and so it is a duplicate of the
one being probed.

Moreover, to prevent acpi_video_bus_probe() from processing two devices
concurrently, which might defeat the above check, use a new internal
mutex in it.

Also, print the FW_BUG message only if allow_duplicates is unset which
allows the entire duplicates check to be skipped in that case (doing
it just to print the message about the case that is going to be
ignored anyway is kind of pointless).

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/3058492.e9J7NaK4W3@rafael.j.wysocki

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