drm/doc: Remove unused Broadcast RGB Property

The Broadcast RGB property has been documented as three separate entries
so far, each with a different set of values.

The first one is pretty much the generic one: it's used by i915 and vc4,
and is the one used by all the compositors.

The second one is used by the gma500 driver, and is a subset of the
first one: it can have the values "Full" or "Limited 16:235", but lack
the "Automatic" value.

The third one however isn't used by any driver and documents the values
"off", "auto" and "on".

It's unclear where the last one comes from. It was first documented in
commit 6c6a3996 ("Documentation: drm: describing drm properties
exposed by various drivers") which adds a number of properties used by
drivers, but without mentioning which driver was using what property.

Grepping at the 6c6a3996 commit however, it looks like no driver is
actually using it, and a quick look at the entire kernel history doesn't
show any match either.

At the time though, gma500 had an "audio" property used right next to
Broadcast RGB that did have the "off", "auto" and "on" values in the
same order.

As such, it was probably a copy/paste or scripting error back then, and
there's never been such property used in the kernel.

Either way, it certainly hasn't been used in a decade or two so we can
just get rid of it.
Suggested-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240527-kms-hdmi-connector-state-v15-17-c5af16c3aae2@kernel.orgSigned-off-by: Maxime Ripard <mripard@kernel.org>

Documentation/gpu/kms-properties.csv

@@ -38,7 +38,6 @@ i915,Generic,"""Broadcast RGB""",ENUM,"{ ""Automatic"", ""Full"", ""Limited 16:2
 ,,“dot_crawl”,RANGE,"Min=0, Max=1",Connector,TBD
 ,SDVO-TV/LVDS,“brightness”,RANGE,"Min=0, Max= SDVO dependent",Connector,TBD
 CDV gma-500,Generic,"""Broadcast RGB""",ENUM,"{ “Full”, “Limited 16:235” }",Connector,TBD
-,,"""Broadcast RGB""",ENUM,"{ “off”, “auto”, “on” }",Connector,TBD
 Poulsbo,Generic,“backlight”,RANGE,"Min=0, Max=100",Connector,TBD
 ,SDVO-TV,“mode”,ENUM,"{ ""NTSC_M"", ""NTSC_J"", ""NTSC_443"", ""PAL_B"" } etc.",Connector,TBD
 ,,"""left_margin""",RANGE,"Min=0, Max= SDVO dependent",Connector,TBD