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The i915 driver has a property to force the RGB range of an HDMI output.
The vc4 driver then implemented the same property with the same
semantics. KWin has support for it, and a PR for mutter is also there to
support it.

Both drivers implementing the same property with the same semantics,
plus the userspace having support for it, is proof enough that it's
pretty much a de-facto standard now and we can provide helpers for it.

Let's plumb it into the newly created HDMI connector.
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Sebastian Wick <sebastian.wick@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240527-kms-hdmi-connector-state-v15-18-c5af16c3aae2@kernel.org

Signed-off-by: Maxime Ripard <mripard@kernel.org>