media: v4l2-ctrls: Allow unknown HDR10 white point and luminance

SMPTE ST 2086 defines the nominal ranges for mastering display
chromaticity and luminance values. Its Annex A also documents that
CTA 861-G uses zero maximum and minimum luminance values to signal
that the corresponding values are unknown, and the xy chromaticity
coordinate (0, 0) to signal that the white point chromaticity is
unknown.

The V4L2 HDR10 mastering display compound control currently rejects
these values. Consequently, an unknown white point or luminance value
prevents the entire compound control from being updated, making the
other valid mastering display metadata unavailable to userspace.

Accept (0, 0) as an unknown white point and zero as an unknown maximum
or minimum mastering luminance. Continue to reject partially zero white
point coordinates and non-zero values outside the nominal ranges.
Display primary validation remains unchanged.

Document the newly accepted unknown values in the V4L2 userspace API.

Fixes: 1ad0de78e794 ("media: v4l: Add HDR10 static metadata controls")
Cc: stable@vger.kernel.org
Signed-off-by: Ming Qian <ming.qian@oss.nxp.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>

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