drm/bridge: lt9611: Add atomic_get_input_bus_fmts

Implement .atomic_get_input_bus_fmts callback, which sets up
the input (scanout-engine-end) formats, so that those formats
can then be used in pipeline format negotiation between this
bridge and the scanout engine.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Dave Airlie <airlied@redhat.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Maxime Ripard <maxime@cerno.tech>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220331153923.14314-2-marex@denx.de