drm/bridge: Add the necessary bits to support bus format negotiation

drm_bridge_state is extended to describe the input and output bus
configurations. These bus configurations are exposed through the
drm_bus_cfg struct which encodes the configuration of a physical
bus between two components in an output pipeline, usually between
two bridges, an encoder and a bridge, or a bridge and a connector.

The bus configuration is stored in drm_bridge_state separately for
the input and output buses, as seen from the point of view of each
bridge. The bus configuration of a bridge output is usually identical
to the configuration of the next bridge's input, but may differ if
the signals are modified between the two bridges, for instance by an
inverter on the board. The input and output configurations of a
bridge may differ if the bridge modifies the signals internally,
for instance by performing format conversion, or*modifying signals
polarities.

Bus format negotiation is automated by the core, drivers just have
to implement the ->atomic_get_{output,input}_bus_fmts() hooks if they
want to take part to this negotiation. Negotiation happens in reverse
order, starting from the last element of the chain (the one directly
connected to the display) up to the first element of the chain (the one
connected to the encoder).
During this negotiation all supported formats are tested until we find
one that works, meaning that the formats array should be in decreasing
preference order (assuming the driver has a preference order).

Note that the bus format negotiation works even if some elements in the
chain don't implement the ->atomic_get_{output,input}_bus_fmts() hooks.
In that case, the core advertises only MEDIA_BUS_FMT_FIXED and lets
the previous bridge element decide what to do (most of the time, bridge
drivers will pick a default bus format or extract this piece of
information from somewhere else, like a FW property).

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed by: Jernej Skrabec <jernej.skrabec@siol.net>
Tested-by: Jonas Karlman <jonas@kwiboo.se>
[narmstrong: fixed doc in include/drm/drm_bridge.h:69 fmt->format]
Link: https://patchwork.freedesktop.org/patch/msgid/20200106143409.32321-5-narmstrong@baylibre.com

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